Black water tank: Every second dump add a half cup of Borax dissolved in 4 pints (a kettle full) of hot water then add half cup of Dawn.
Grey water tank: Every second dump add a half cup of Borax dissolved in 4 pints (a kettle full) of hot water then add half cup of Zep (Heavy duty Citrus degreaser & cleaner)
Never had sensor problems.
Someone said they use 2 big bottles of Coke and leave for a day then all sensors work ok again.