The problem seems to be caused by sludge or film that forms on the sides of the tanks that causes the sensors to conduct even when the tanks are empty. Maybe the fact they don't work right is the way for us to know we need to do a better job of cleaning the tanks. But sometimes we want to conserve water. Oh well.
Here's a couple of links that help describe the problem and a redesigned product (Horst miracle probe) that allows these sensors to be more reliable. I think there have been a couple of members who have posted success with them.
http://horstmiracleprobes.com/ &
http://www.rvprobes.com/buyprobes.html
Other than the difficulty of installing them and it's extra cost (slightly cheaper in volume) I wish the RV manufacturers would use them since the existing technology is so unreliable.