The secret of tank cleaning (and it is more than just sensors) is prevention.
The liquid Calgon Water Softener (not fabric softener!) and detergent treatment in each tank WORKS.
Once there is slime on the tank walls only a thorough cleaning with a cleaner made for tank sensors will give you a hope of cleaning the tank walls of conductive slime.
I solved my problems by replacing the button sensors with sensors that have a Teflon collar the does not allow sensor to WALL contact and only will complete the circuit through the liquid.
Horst tank Probe Installation
and here is some additional information on gray tank cleaning:
Troubleshooting the MicroMonitor
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW