Sounds like the gauge is stuck, but perhaps not the float.
If the float is magnetically coupled to the gauge, then (if I'm reading this correctly), one can remove the 2 screws holding the dial in place and remove the dial w/o emptying and depressurizing the tank, correct?
If that's the case, then the gauge can be removed to check to see if the dial indicator is seized. If not, then perhaps the float is stuck.
(Thread sidetrack...)
Is the dial keyed to the fitting? Mine never hits full, though the marks between 3/4 and full is about a 10th of the rest of the gauge.
I'd like to rotate the dial just a bit so that it actually reads full when it is full. I understand the need for the airspace for expansion, but I feel that the gauge should read full when the tank is as full as it can be.