I think all tank sensors are not to be believed. If you fill the tank and monitor the sensor to the point it reads F then shut the water off, it will read F for a few hours or until you move. Then it will read 2/3. Then it might go back to F. If you fill the tank until the overflow starts, you will see about 2 gallons of water (out of a 100 gallon tank) spill out, but you will see the gauge read 2/3.
Yep there is a bubble around your Newmar that invalidates all the laws of physics