I had a thread on the Class C forum the other day asking about all that. It turned out that the gauge on the tank itself reads 80% or about 3/4 on that gauge, when tank is "full" (as it should) but the monitor level panel idiot lights then shows "full"
Somebody said when the monitor idiot light level does not match what it says on the little gauge on the tank itself, there is a way to line them up using a magnet. Apparently RV techs know how to do that.
The story came out that the little gauge on the tank is way more trustworthy than the monitor level lights.