As Donn says, check the probe for voltage first; there will only be voltage present while the button is depressed. Use TSP cleaner in your tanks to clean them out. Dissolve the appropriate amount for your tank in a gallon of hot water, pour into tank, then fill to 3/4 (ya you gotta guess) with clear water. Drive around (or to your next nights stop) to slosh the cleaning mixture in the tank, then allow to sit overnight - if practical.
If you're considering upgrading to a reliable tank monitoring system: See-level gauges
have a good reputation in the RVing community.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"