I don't depend on my tank monitors much... but do use them to get an idea of how full they might be.
I have found that the black tank monitor will read "full" immediately after flushing the toilet, even if I just emptied the tank. A few minutes later when "stuff" settles (I guess) the monitor will read less.
If your toilet empties directly into the black tank (no bends in piping), the best way to know for sure how much is in the tank is to look down the toilet using a flashlight. Also remember that the tanks are very shallow but wide, not deep, so it may look like it is filling up but you probably still have plenty of room.
Have fun! and best not to tell non-camper friend what you are doing, unless you want to get the funny looks from them!