Black holding tank censors

Those "sensors" are merely bolts thru the side of the tank, not some fancy electronic device. They do not need to be clean, the problem is the scum on the tank wall bridges the space between the bolts and allows current to flow. The system works by applying voltage to the bottom bolt and water completes the circuit. The more bolts reached by water, the higher the gauge reading. Keeping the tank wall clean will allow the gauge to work properly, but that is nearly impossible. If you use chemicals, soaps, detergents, water softeners or anything else, remember that they will increase current flow so the tank needs thorough rinsing with clean water after use, just like washing anything else.