Hey Skipat, The sensors are notoriously fickle. All it takes is the wrong piece of food waste gumming up the sensor and you will get a false reading. The black tank is even worse, owing to the TP that gets loaded in there.
There are some chemicals out there which will help break down waste inside the tanks and can help to keep your sensors clear. But I have found it is simply easier to not trust them and rely on my own knowledge of water use in the rig. Dry camping is easy because most rigs have black/grey tanks which are no bigger then your fresh water tank. Full hook up sites are also pretty easy, I empty my grey tank every two days and the black every three or four depending on use. The real challenge comes with partial hook up where you are connected to water but not sewer... That is when I like to use those chems and try to keep my sensors free of debris, I generally know that my family is good for four days with partial hook ups so I try to limit my stays to that length.
Griswald One - Life is a Misadventure!
2015 Greyhawk 29ME / Chevy Sonic