I assume you mean they are never accurate, and not that there is a short in the wire and you get no level reading. Unfortunately this is a common problem.
For the black tank, most of the time TP, builds up on the sensor probes and they stay wet, and give a false fuller reading. You can bring a hose into the camper and use a "wand" and rinse the tank, usually this helps. On a 5ver it might be more challenging. Some people swear by putting a bag or two of ice cubes down the drain and go for a drive. I personally like to use a water wetting agent. I use liquid fabric softener, others use "water softener" Basically this allows the water to more easily soak into the ick, and help it dissolve the matter. I use about a cup of fabric softener to 4 gallons of water. Poor about 1/2 the solution down each my black and grey tanks. I will do this about 3 times, after I dump and before I drive home, and let it slosh. Make sure to have plenty of fluids int the tank so it can slosh nicely.
Grey tank has a similar issue, but instead of TP, I think the issue is grease buildup on the sensors and tank walls. I use the same mixture to clean the tank. I never use bleach as it kills the good bacteria that keeps the odors down.
Also very important after dumping your waste tanks, make sure have enough water in them. In the Black tank add 10%+ of tank volume back in, so a 50 gallon tank should have about 5 gallons or more water put back in. The grey tank, I always keep a few gallons in mine.