Do RV holding tanks have an overflow feature?

In our years of servicing RVs, we have yet to see any make or model that has an engineered and planned overflow feature to either the black or gray tank. That being said, there are ways that your holding tanks can indeed overflow.

Your black tank typically has three connections to it: The toilet, sometimes accessed via a direct drop into the tank or by a pipe from the toilet to the tank; a vent pipe that tends to be located at the top of the tank and travels up through your RV to the top where there should be a cap on it; and the waste pipe that allows the tank’s contents to be emptied.

The gray tank likely has the same number of connections with multiple collection points (e.g. a sink, the shower, etc.) gathering into one pipe before it enters the gray tank.

If your tanks are beyond capacity, a backup will likely happen in the toilet for the black tank and in the shower for the gray tank. These are the lowest points in the plumbing system for each tank. In some cases, the toilet’s scissor-like release valve at the bottom of the toilet bowl may not allow any waste to actually fill the bowl. In that case, the waste from your black tank finds another way out….

We have never seen — though certainly have heard rumors of — tanks backing up into the vent pipes, emptying out onto the rood of the RV. This most commonly happens when an RV owner leaves their black tank rinse system unattended with the release valve closed. In this case, the tank fills beyond capacity and waste travels up the vent pipe.

Smart tank maintenance starts with tank monitors that function properly and are watched to see how full the tanks are. Unfortunately, most tank sensors no longer work as a result of the tank additives RV owners add that foul up the sensors. In these cases, you’ll have to work out an estimate as to when your tanks are ready to be emptied based on usage.