We had a similar problem with our tanks, we put some chemicals found at most RV stores or even at Wal-Mart, into the sinks and black water tank these are intended to break up grease and soap scum, I added 6-8 gallons of water and drove to our next destination! With the coach moving it caused the water to move around and it apparently helped to desolved the problem and we got back to normal readings, we found that it helps to do this now and then and it keeps the probes working.
Good luck,
Mike