How an RV Toilet Works: Everything You (Didn’t) Want to Know

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I wanted to begin this article with a poop joke, but they all stank.

Dad Joke Meme Picture

Alright, let’s get serious. We aren’t 5th graders (geez). Now, here’s the big question: How does an RV toilet work?

This isn’t an unimportant question. I know life-long campers who are afraid to poop in their own RVs. They don’t understand the system. And that’s a shame. No one should be so scared of a mechanical device that they can’t go Number Two.

How do RV toilets work? Well, before we dive in – hmm, should have phrased that differently – here’s some terminology you should know.

Meet the RV Tank Triumphate

What Is an RV Fresh Water Tank?

It is what it says it is. This tank holds the clear, clean, life-giving liquid you use to drink, bathe, clean dishes, fill up water balloons, hose down the dog, etc.

You fill up this tank using unpressurized water through a port in your sidewall called a “gravity fill.” A water pump pressurizes the water and delivers it to your faucets, showerheads, etc.

This is NOT the same thing as hooking up to a campground water supply. We call that “city water,” and it’s pressurized anywhere from 25-75 psi. Your campground should really keep the pressure around 40-50 psi, but they often don’t, so you should use a pressure regulator to protect your system from high operating pressures.

What Is an RV Gray Water Tank?

A gray water tank holds the dirty water from your sink and shower drains. It is designed only for liquid waste only! The system uses 1-½-inch pipes running horizontally and vertically. So don’t try to wash potato peelings or large chunks of food down the drain. Solid wastes will get stuck in the system and lead to an expensive repair bill.

What Is an RV Black Water Tank?

Meet the notorious black water tank.

I’ll let you imagine where the name comes from.

This is a holding tank for human effluent. It’s designed for solid waste and degradable toilet paper. The drain piping is usually 3 inches and has a minimum horizontal slope of 45 degrees.

Sizes for all these tanks range from 5 gallons for a teardrop camper to 120 gallons for a motorhome or fifth-wheel. Usually, your gray water tank is largest, followed by your freshwater tank, and lastly by your black water tank.

Some RVs have a combined black and gray water tank. This isn’t necessarily a deal-breaker, but you’ll need to dump a LOT more often – and remember, you can only dump a black tank at an approved RV dump station.

Introduction to the RV Body Waste System

The black water tank and its gang are our subjects. So let’s break them down.

Just like your –

Sorry, sorry. I promised. Anyways … so how does an RV toilet work, again?

Walk into any run-of-the-mill camper, and you’ll find what’s known as a “mechanical seal” or “gravity” toilet.

We engineers aren’t known for our creative naming skills, so a gravity toilet is exactly what it sounds like. Waste enters and exits the system because of gravity. Everything travels downwards!

This sounds simple, but this is where a lot of RV owners get confused. After all, in your home, you just flush the toilet, and everything disappears with a roar, and you never have to think about it again.

Unfortunately, we don’t have septic tanks or unlimited water supply in an RV. So we have to get a little creative.

How Does an RV Toilet System Work?

When you use the bathroom in your RV, all your “stuff” gets stored in a tank. When the tank fills up, you drive to an RV dump station, attach a 3- or 4-inch wide flexible hose to your waste valve, open the system gate, and flush the dirty wastewater into an approved dump receptacle.

It’s not as gross as it sounds (but it still is a little gross). Why the RV industry hasn’t moved to an industry-standard park n’ dump system, like the International Space Station dock, I have no idea. It needs to happen. But for now, we’re stuck with sewer hoses.

Here’s a super-simple diagram to help explain things.

There’s a lot more to the drainage system – vent pipes, air admittance valves and all that – but this really is as simple as it gets: from toilet to tank to dump station.

So your typical RV toilet has one job: Make sure waste goes down and nothing – no waste, no smells – ever comes up.

What if I asked you to design an RV toilet. How might you do it? Maybe something like this.

  • Make a bowl.
  • Drill a hole in the bottom of the bowl.
  • Attach a tube to that hole.
  • Oh, and put a little valve in the hole that opens and closes with a lever.
  • Put a lid on the bowl.

Congratulations, reader. You’ve just invented the RV toilet. Genius, sheer genius!

A gravity RV toilet really is that simple. Here’s how it works:

  1. You “use the premises,” as they say.
  2. Fill the bowl with some water. Generally, you push the lever halfway to fill the bowl without opening the valve.
  3. Flush the bowl. Push the lever all the way, and the flush ball valve opens. Goodbye, lunch!
  4. Ease up on the flush lever. The valve closes and a small puddle of water collects on top, sealing off the tank smells from your RV interior.

Now I know what you’re thinking. “Surely there’s more to the fearsome RV toilet than this?”

Well, not really. I’ll talk about some feature options in a minute, but that really is how 90% of RV toilets work. Your doo-doo drops directly into the black water tank, and from there it’s a straight shot out the sewer hose to the dump receptacle.

RV Toilet Feature Comparison

If you’re shopping for an RV toilet, here are some features to keep in mind.

Compact vs Full-Size RV Toilets

RV toilets don’t really deserve the title of The Porcelain Throne. More like the Plastic Foot Stool, or Plastic Doll Chair. They are quite small. You want one that’s “residential-sized” if possible.

RV toilets do not all have a standard seat height! I’ve seen as tall as 21 inches and as low as 13.5 inches.

Toilets shorter than 15 inches are normally called “compact” or “low-profile” toilets, and they’re designed for powder room bathrooms with a raised platform.

You want to aim for a seat height of 16-18 inches.

Plastic vs Porcelain Construction

Once upon a time, almost all RV toilets were made of cheap plastic. That’s not the case anymore. Many toilets, even value options like the Dometic 310, have a plastic body with a ceramic bowl. This really is the best of both worlds. China porcelain is heavy, folks.

Luxury toilets, like macerator or all-electric models, usually have full ceramic bodies.

Only the cheapest units or porta-potties still have all-plastic construction.

RV Toilet Water Efficiency

Small RV toilets use as little as one pint per flush. Others might use as much as half-a-gallon.

(For reference, the U.S. standard is one gallon per flush.)

Now, water efficiency is great – to a point. Hate to be gross, but the waste in your holding tank needs to be liquid in order to flow.

If you don’t use enough water per flush, or if you refuse to pee in your toilet, then you’ll likely run into two problems:

  • You’ll clog up your tank. This normally means a heavy-duty flush and possibly removing the waste valve for tank body access.
  • You’ll get the dreaded “mounding” problem. Imagine a poop pyramid rising into your toilet, and you get the basic idea.
Empty Toilet Paper Holder Dont Panic

Round vs Elongated Toilet Bowl

What you want is a deep, elongated bowl. If you can’t figure out why, please go ask someone else. I don’t want to explain it. Or just take my word for it.

Shallow round bowls pose a couple of obvious problems. For one, you need, uh, space for your waste! And secondly, for men, an elongated bowl helps prevent splash out if you urinate while sitting down.

RV Toilet Flush Pattern

A little dribble of water from a single spout isn’t going to clean the whole bowl, now is it? Manufacturers now (mostly) use full-rim flush patterns that wash the entire bowl. Some toilets even come with handheld sprayer wands if you need a little extra cleansing. Chile, anyone?

Salespeople like to give these designs superhero-esque names: the vortex, 360-degree, swirl-jet, waterfall. Don’t get caught in the lingo. You just want a toilet that rinses the entire bowl, not just the back.

Hand or Foot Lever

Some toilets use a hand flush lever; some use a foot flush lever. One is easier; the other is more sanitary. Choose whatever you prefer.

Lever vs Button

You might not have to choose a lever at all! Like a residential toilet, electric RV toilets offer a similar flush-and-farewell experience. You flush using an electric button or a remote wall switch.

You’ll pay dearly for the upgrade, however, and you’ll need to route wire to the toilet if you’re remodeling.

White vs Wood Lids

RV toilet lids come in all colors – so long as they’re white. Some are plastic; some are ceramic. A few models come with standard wood surrounds.

If you don’t like yours, the aftermarket industry sells a bazillion toilet seat covers and lids. You can even buy a fuzzy, plush blue toilet seat lid!

What Is the Best RV Toilet?

This, dear reader, I cannot answer for you. The needs of a pop-up camper are completely different from a fifth-wheel. Just remember this: You get what you pay for.

Now, why are there not more options for RV toilets, you’re wondering?

Well, there aren’t exactly dozens of toilet manufacturers trying to out-think the other. There’s basically two: Dometic and Thetford.

I’d hazard a guess those two companies own 95-plus percent of the OEM RV toilet market.

RV Toilet FAQs

Alright, I lied. Actually, I’m saving my RV toilet and black tank FAQs for another article. But I do want to answer two EXTREMELY common questions:

1. Do I have to use special RV toilet paper?

No, you don’t. But that doesn’t mean you can just buy your favorite triple-ply Cottonelle and wipe with abandon. From an engineering perspective, the thinner, the better.

Toilet paper needs to break down. Otherwise, it’ll clog. Take a sheet of your preferred toilet paper, put in a glass of water, and wait. If it hasn’t broken down in 2-3 minutes, find something else.

Don’t flush anything you don’t have to. You can flush No. 1’s and No. 2’s with limited amounts of toilet paper. Anything else? No.

No diapers, napkins, feminine hygiene products, paper cups, trash, etc.

2. Why do RV toilets leak?

If you’re asking why RV toilets leak effluent waste, that’s normally a failure of the gasket, flange seal or wax ring between the toilet base and the floor. The only solution to a leaky base is to re-install the toilet body.

If you’re asking why RV toilets leak odors, that’s usually because of a failing flush ball valve. Dometic and Thetford both warn users never to scrub the ball valve or use harsh cleaners.

Even so, over time, hard water deposits can build on the valve seat, and you’ll start to smell odors escaping from the black water tank.

A leaky ball valve can normally be fixed with a good cleaning or valve replacement.

The Runners Up: Alternative RV Toilets

I’d like to award some honorable mentions to toilets that didn’t get mentioned in this article. We won’t be discussing these toilets today, but you should be aware of their existence:

Portable RV Toilets

Portable toilets come in 1-gallon to 5-gallon sizes, but they all work on the same idea: poop now, discard later. Some use natural fillers like sawdust to mask smells and break down waste; most use chemical treatments in liquid or powder form.

Composting RV Toilets

These Mother Earth miracles use microbial activity to break down human waste in a matter of weeks – or even days! They usually separate liquid and solid wastes and require a 12V electric hookup for ventilation and heating. They are a favorite amongst boondockers.

Macerator RV Toilets

Imagine a garbage disposal for your personal, uh, garbage. These toilets have grinding blades and a pump that turn brown solids into a viscous slurry that can be pumped through smaller pipes into distant holding tanks.

Vacuum RV Toilet

Often used in combination with a macerator system, a vacuum pump powerfully pulls the bowl contents through a vacuum vessel and macerator pump before storing the waste in a black water tank. This system is seen on campers where the toilet needs to be located far away from the waste tank.

Incineration RV Toilet

These are so badass! They incinerate your poop at about 1,400 degrees Fahrenheit. That’s hot enough to melt your aluminum cookware!! But at about $2,000 a pop, you won’t find these in many RVs.

Cassette RV Toilet

Hailing from Europe, cassette toilets are all-in-one toilet+tank systems. From the inside, everything works the same. From the outside, it’s all different. Instead of attaching a sewer hose to the waste valve, you slide out the entire black water tank. It’s self-contained, and usually has wheels. Then you tow the tank to a dump receptacle.

In conclusion, I leave you with this epic, cringe-inducing, terrifying scene from Robin William’s underrated film: RV: The Movie

Now, I’ve never heard of this actually happening.

But if it’s happened to you, please submit your full story – complete with photographic evidence – and I’ll give you a $100. No sh*t.

Ross

RV engineer by day, intrepid blogger by night (and occasionally weekends). This website is all about how RVs work, and sometimes why they don't. Bookmark pages that you find helpful, and join my email list for exclusive monthly awesomeness.