In a 23 ft travel trailer for about 6 months and the black water and grey water sensors are stuck at 2/3 for the last month. We tried some of the granular stuff from camping world but didnt do a thing...
Every few years I will use a bottle of septic tank cleaner and top off the tanks with water. I let it sit for a week or two and it seems to do a good job at dissolving everything in the tank. Probably takes longer if you try this in cold weather.
Old butt wand usually works on Black water tank - and using dawn will help with grease on gray water tank. Most of use tend to ignore the sensors.