youtubeThis article is another topic on the list of things your dealer may not have told you about how to use your RV. Possibly the least favorite all-time jobs an RV technician has to deal with is a problem with the holding tanks, specifically the black (toilet) tank(s). Working on holding tanks is a difficult, time-consuming, and messy job. If you have ever looked at the waste plumbing on the outside of your RV, you may have noticed that it consists entirely of rigid ABS piping, the same as your domestic drain, waste, and vent (DWV) plumbing.

Additionally, the gray and black tanks share a common exit point where the bayonet fitting that you attach your drain hose is. Some RVs have the gate valve assemblies accessible under the frame, while others have the valves themselves behind the belly material, with the handles protruding out the side of the vehicle. Still others have the valve assemblies mounted inside climate-controlled compartments to prevent them from freezing in the winter. The point is, repair on the waste plumbing system is agonizing for the RV tech and potentially expensive for the owner.

Black Valve & Bolts

Exposed Waste Plumbing

Gray Valve Bolts

Twin Dump Outlets

Dumping Valve Assembly

Valve in the Underbelly

Waste Dump into Underbelly

Waste Valve with Cap

But why would the waste plumbing system require repair in the first place? There are a number of reasons, for instance, it is possible that the valve mechanism may become damaged. For example, the handle shaft may become separated from the gate valve, but this is not common. The seals may fail or become damaged, resulting in a leak. Waste may become trapped in the valve, resulting in the gate valve not closing all the way. Also, the black tank may become clogged with waste, resulting in drainage problems. The latter is probably the most common, so let’s discuss the proper method of dumping and maintaining holding tanks so there is less chance you will have waste plumbing issues.

If you are at an RV park with full hookups, you can connect your sewer hose and leave it connected for the entire visit. Ideally, it should be supported off the ground, and in some states and provinces it is the law. You may leave your gray valve open while the hose is connected to the park sewer, but consider closing it the day before you leave for reasons I will explain shortly. Do NOT leave the black waste valve open. Doing so results in the liquid waste draining into the sewer, but the solid waste will drop down into the tank. Without any liquid, the solid waste can’t exit the tank. Leaving the black waste valve open consistently usually results in dry, hard piles of solid waste building up in the tank. This effectively takes the consistency of concrete and is exceedingly difficult, if not impossible, to clean without removing the tank. This waste will usually pile up right under the toilet, eventually causing an issue with flushing. It may also ultimately result in issues with dumping the tank, as the waste may impede flow through the gate valve. Therefore, it is important to always ensure there is liquid waste in the black tank to suspend the solid waste.

Whenever possible, do not dump the black tank until it is at least 2/3 full. This will provide the best chance for a complete evacuation of the tank, caused by the vortex created by the large volume of waste exiting the tank.

Always dump the black tank first, then the gray. This will allow the gray water to rinse the black waste out of the sewer hose. This is why I mentioned earlier to try to have at least ¼ gray water in the tank when you need to dump the black tank. Even better, a black tank flusher is recommended, so if your RV didn’t have one factory installed, you might want to consider having one installed aftermarket. While they won’t help much if you already have a mound of solidified solid waste, they are helpful for preventing it in the first place. Regular flushing of the black tank, even if it is through the use of a flush wand that you put down the toilet, is a great way to help keep the tank clean.

A good argument for leaving both valves closed until you need to dump is sewer gas. If you leave your valves open, odors from the park septic system may flow up your sewer hose and into your RV. The water traps and water in the toilet should prevent this, but if you lose your water traps, you could experience sewer odor.

If you have a clothes or a dishwasher in your RV, it is important that you keep your gray valve open, so whether you keep your gray valve open or closed will depend on your personal requirements. However, it is important to keep your black valve closed unless you are dumping the system, in which case it should be at least 2/3 full.

These best practices will help keep your waste plumbing system functioning problem-free.

Steve Froese
Author: Steve FroeseEmail: This email address is being protected from spambots. You need JavaScript enabled to view it.
Steve Froese is a Licensed Professional Engineer in British Columbia, as well as an Interprovincial Red Seal RV Technician, which is equivalent to a Master Certified RV Technician in the USA. Steve was a personal friend and colleague of the late Gary Bunzer (“the RV Doctor”), and works closely with FMCA as the monthly “Tech Talk” columnist, as well as being a member of the Technical Advisory and Education Committees. Steve and his family are lifelong and avid RVers, mostly in the Pacific Northwest.


+1 #1 Jean Chodakauskas 2021-01-30 16:39
Hi Steve,

Thank you for your great articles. I’print and add them to our RV notebook under tips and tricks.

As new RVers you articles give us greater self-confidence to hit the road and enjoy RVing year round.


Jean Chodakauskas

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