Fireflipper
BigBlue's suggestion about trying to keep the tanks clean is right on the money. I had the problem with the black tank, after time, and it does not take a long time, stuff will build up on the sensors. I use a product called Orange Blast, from Walmart. Whenever I hit the road I put about a guart in each tank and about five gallons of water. While traveling it gets slushed around and keeps the tanks and the sensors clean. Have not had a problem for awhile.