My kitchen gray tank sensors read full for over a year regardless of full or empty. A friend suggested I empty the tank and put a few gallons of water in it to allow sloshing around while traveling and add one full gallon of bleach. 230 miles later I emptied the tank after arriving back at my home base and the sensors all worked perfectly like new. I now add a little bleach to this tank after each dump, but only a few ounces just to keep things clean. I will be traveling 250 miles in mid-May and I'm going to try this with my black tank. These sensors have registered nothing but full for the past 4 years.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof