Well, false readings are normal and to be expected. If you don't have them, then is definitely something not right! :B
Some tanks are easy to dump and flush and some aren't because they come in many different shapes/configurations and dimensions and the outlet can be on the side, end or bottom. We've had both extremes and currently have the difficult type because it is too shallow compared to it's width & length. There is the Horst Miracle Probe that may or may not help. The Horst probes really helped on a previous TT but did almost nothing in our current TT. Another thing that can happen is that the factory sometimes installs the button sensors in incorrect elevations and they won't read right no matter what. For more a lot more $$ you can get the SeeLevel sensors that go on the outside of a tank. If you have an enclosed underbelly, changing sensors is not a simple job tho.
Preventive measures will help for the black tank - 3-5 gallons of water before use, fill tank to near full before dumping and flush/rinse as best you can before heading off. A built-in flusher works well for us (can be retro-fitted). A Valterra backflush elbow worked great on a previous TT but does nothing for our current one. If the toilet pipe drops straight down to the tank, a wand type flusher will help.
If you have a really plugged up tank or it's walls are coated with slime and short-circuiting the sensors, the best method of all for cleaning is Liquid Drano (or equiv.). Our owners manual actually says to use it. It needs to be the type that doesn't not attack metal and do NOT dump it into a private on-site septic system. We've used it 2 or 3 times and it is amazingly clean afterwards. Sadly, it doesn't take long for false readings to come back. :M