If you don't get false readings, then something is really wrong, lol... There's many thousands of posts on this topic if you do a search- plenty of reading enjoyment! Slime covers the tank walls and short-circuits the sensor surfaces. Not the easiest or quickest of jobs to clean them.
First, make sure to add a few gallons before each use of the tank after dumping, don't skimp on water with each use (esp. no.2) and fill the tank to or near full before dumping. Sometimes it helps to do a second or third filling of the tank to flush more stuff out. After finishing dumping, use a rinsing method of your choice - built-in rinser, back flush elbow or down the tank wand. Don't rush the rinsing process. I let our in-tank rinser run for something like 15-20 minutes when breaking camp and attending to the other chores.
Then everyone has their own choice of tank chemicals. Some use the "geo method" to clean sensors (google it for details). There are some commercial cleaners (Thetford, etc.) but needs to be warm out. Our owners manual says to use Liquid Drano. I do that once or twice a year but the sensors stop working properly shortly afterwards. With any cleaning chemical products, you need to drive around to make the contents slosh around but not possible if set up on a site for a long period.
Most simply keep tabs on the number of days the black tank has been in use and/or peer down the toilet. Horst Miracle probes are an option and easy to install if you have access to the tanks. We installed them on a previous TT and they worked great but on our current TT don't help at all. (Depends on your particular tank configuration.) The only reliable sensors are the SeeLevel ones that are external.
