Black tank sensor fixes itself: cure or remission?

On our RV, the fresh, grey and black tanks have external sensors that work the way an electronic stud sensor does. When the water in the tank is in proximity of the sensor, the control panel displays the reading. I would think that this should be standard by now as the old contact type are unreliable. Even in the fresh water tank on my old rig was inaccurate because some water sources have less mineral content causing false readings. Unfortunately, the circuit in my control panel for the black tank failed and always reads empty. Rather than replace the unit or attempt repair of the circuit board, I just shine a flashlight down the toilet that sits just above the holding tank. A friend has a fifth-wheel that is only a couple of years old and he is always having to fiddle with the black tank to get readings. I think it is nuts to have to buy bags of ice cubes and drive around a bit to clean the sensors. The external flush connection is useless. I have yet to find a tank additive that is effective either.