Over time the walls of your RV waste tanks can get a build-up of sediments, which can falsify the sensors on the side of the tanks. Inaccurate readings caused by residue and stuck toilet paper on the walls of the RV waste tanks is a common problem owners regularly face.
Most systems have a main probe on one side and three or four several inches to the side. As the fluid level rises inside the tank, low voltage passes through to the other probes and indicates the various levels. When residue builds up on the sides of the walls, electrical continuity is still present so the probes send as if the tank is full when it is not. Even after completely dumping the RV waste tanks, you can still get a false reading from left over residue and items stuck to the walls where the probes are coming in.
If you are getting false readings, there is an inexpensive and easy way to conduct a quick cleaning of the black water tank walls. Take four to five of the five-pound ice cube bags and dump them down the toilet. The ice cubes will bang around the tank cleaning the sides and probes before they melt. Make sure to do this while driving, otherwise it will not be effective. The longer you drive the better. Once done, the water can be dumped down into a dump station.
Using ice cubes is a quick and easy way to clean residue off the side walls of your RV waste tanks. Hopefully doing this will take care of the false readings you are getting without having to spend a lot of money on replacing the probes. However, if cleaning does not work, it may be best to contact a certified mechanic.