Black and grey water can develop a film that coats the sensors and leads to false readings.
We had our last travel trailer for eight years. Early on we started using the Geo Method
. We didn't have a black tank flusher on that trailer so two or three times a season I used a tank wand through the toilet to spray the interior with clean water. Our sensors were still working properly when we sold the trailer this spring.
Our new-to-us Rockwood 2701SS is a 2012. Initially the sensors were not reading correctly but after a couple of trips, a couple of doses of the Geo Method's Calgon/laundry crystal mixture and time spent flushing the black and back filling the grey tanks, they're much better.
One thing I found helpful was to run the black tank sprayer with the drain valve closed for about thirty seconds, then drain the tank. Each time I did that some waste was flushed out. I repeated the sequence until the water was (relatively) clear.
It does help that we can dump at home with the help of our Flojet... I have access to all the time and water I need.