In my manual that came with the MH regarding holding tanks it stated that something as simple as: "certain minerals found in well waters we get from CG's, our own wells and city water can foul the sensors."
Just using soaps etc will not remove the minerals from the sensors.
After reading this I started using the Thetford Sensor Cleaner that they suggested once a month. More if I was using different CG waters on my trips.
My sensors all work all the time! :C
1. Chems in black tank.
2. Qrt of lemon cleaner in gray tank often - to keep food stuffs, etc off sensors.
3. Thetford Sensor Cleaner once a month - to remove hard minerals from sensors.