When your tank gauge reads FULL it is at the 80% level by design. You can see that on the gauge F and 80%.
The fact that you went from 1/2 to F with 4.3 gallons would lead me to believe you have a 40 pound tank, not a 76 pound. The numbers on the tank are key to the mystery.
One of 3 things are going on: 1) the RV builder put the wrong tank in...not unheard of, or 2) the float in the tank was not installed correctly, or 3) the tank gauge is faulty. I would take the gauge off and see if there is crud behind it. It is either a snap on or there are two small screws holding it on. It will come off without losing any gas from the tank.
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