If you’re wondering whether you should leave your RV’s black tank valve open or closed, the short answer is: Closed. You should keep it closed until the tank is ½ to ⅞” full, or you’re getting back on the road after camping. Best not to travel with a full holding tank if you can help it. They’ve been known to fall off! 

When it comes time to dump, ensure your camper is parked on a level surface. Attach your 3” sewer hose (aka “stinky slinky”) to the fullway valve outlet (bayonet fittings are the most popular), then drain the tank all the way. Avoid sags in the drain line. Dump the black tank first, then gray, if you can.

Next, rinse and flush the black tank with 3-5 gallons of water using an aftermarket toilet tank wand (aka “swivel stick”) or built-in black tank rinse, if your RV came so equipped. Check the color of the effluent through a clear sewer hose elbow in your drain line. After those few gallons, the water should run clear. Once that happens, close the sewer valve.

Add an enzyme-based tank cleaner concoction and 2-3 gallons (or as many as your cleaner instructions dictate) of water to your black tank. Make sure your toilet has a little bit of water above the rubber seal. Good job! You’re ready to go again (no pun intended). 

>>> READ MORE: Here’s How Long You Can Camp Without Dumping Your Black Tank

Every 1-3 years, have your gray and black tanks professionally cleaned to break up struvite deposits and hardened sludge. This will help ensure accurate tank sensor readings, increase your tank capacity, and otherwise keep your holding tank clean and not so nasty. 

The 5 Simple Rules to Dumping Your RV’s Black Tank

It’s not rocket science. Unfortunately, there’s a lot of bad advice out there on RV forums. Plus, Google tends to reward web pages with long-form content, so a lot of RV blogs have unnecessarily complicated the simple act of dumping your tanks. If you follow a few simple rules, you’ll avoid 95% of black tank clogs and problems:

1. Keep your black tank closed until full or it’s time to go home. Dump the black tank first, then the gray tank.
2. Don’t be a water miser. Rinse your empty tank with water. Add water before using it again. Flush the toilet with lots of water. Water is your friend! Water makes soup, not sludge.
3. Use enzyme-based tank treatments like Happy Campers or Unique. Put those bacteria to work!
4. If you forget to follow Rule #2, add some chunky ice cubes to your tank before your next road trip. It works like a redneck rock tumbler.
5. Don’t flush anything besides No. 1, No. 2, and single-ply toilet paper. No hygiene products, paper towels, wipes, or goldfish.

If you really want your holding tanks in tip-top condition, have them professionally cleaned every few years. To avoid the potential of sewer gas leaking into your interior, you can even have the waste tanks professionally cleaned every fall before winterizing. Professionals use high-powered, pressurized equipment that will descale the sidewalls of your tank, something a simple rinse can’t do. 

Here’s Why You Keep Your Black Tank Valve Closed Until Full

So, why the Golden Rule to keep your black tank closed until it’s time to dump? Because liquids run faster than solids. A holding tank is supposed to contain a sewer soup – and yes, that’s the consistency you’re looking for. If you leave the drain open, water and urine will drain away, leaving the solids to coalesce into the dreaded “poop pyramid.” Good luck getting rid of that. Hardened waste doesn’t always dissolve, even when you add water (or ice) in the future. It creates clogs and fouls up tank sensors. 

There are other good reasons to keep your sewer valves closed. The campground’s sewer system contains nasty odors, nasty flies, and other unmentionables. A tank filled with poop is bad enough – do you really want one filled with poop and maggots? It’s rare, but hey, it only takes once to ruin your vacation (and give you PTSD). 

Some RVers feel comfortable leaving open their gray tank, but not their black tank. You’re still at risk of sewer flies and odors, but you won’t get clogs from congealing solids. Your call. Others prefer to dump their gray tank after their black tank, since the gray tank water kinda-sorta scours the common drain line. It’s still gross though, so follow up with some clean or lightly soapy water through the gray tank line. 

“Should I leave my black tank open or closed?” Ultimately, you have to accept that the RV sewer system really isn’t designed that well in the first place. Just do your best. Pay a professional to do the rest. Happy camping!