Probe Cleaner, not just for aliens...
A common boat maintenance issue is the holding tank level indicator. Tank level indicators generally work very well in water and fuel tanks, but the holding tank is a larger challenge.
Tank gauges work in a variety of ways; there are float level sensors with a mechanical linkage, electronic sensors that send sonar signals to sense the level and others that measure resistance at different heights. All are sensitive to sludge building up on the sensor and causing erratic readings.
The holding tank and system occasionally need a good cleaning. Remember the old Du Pont tag line "better living through chemistry", well cleaning the tank is a good time to rely on chemistry.
Here are a couple of products that boat owners have offered positive recommendations. TST Probe Cleaner, $12 at Amazon or RV Supply stores. Noflex Digestor can be found at Fisheries Supply or Marine Sanitation for about $19.