The sensors soon get gunked up and fail to report the level accurately. Here is a current discussion on the this topic:
https://www.montanaowners.com/forums...ad.php?t=89273
You will learn how your habits affect the filling of the tanks. The black tank "burps" when it is close to full, the kitchen and shower tanks back up into the sink/shower to say they are full.
Some use a special concoction to help keep the sensors clean, others replace the sensors with an external See Level gauge system.