Full voltage will be full tank, although this may be regulated before it hits the gauge, but I can't remember seeing a voltage stabiliser when I did my bulkhead so I don’t think it is.
Anyway, apply 10volts to it and it will either show full, or about full, if it shows about full then it’s got a built in voltage stabiliser in which case you can give it from 0v – 14.4v happily.
If it’s always showing full then it is almost certainly because the wiring is touching somewhere or the sender in the tank is goosed.