I think your fuel consumption estimate may be a little high because at night air conditioners don't run steady most of the time. Onan specs show .3 GPH at no load and .6 GPH at 50%, which is around 23 Amps. Averaged over one day and two nights you may have used less than you think.
I have no idea why it won't show a decline on fuel gauge -- hopefully others may know. I've driven many cars that when tank is full, it has to be driven quite a bit before gauge moves at all.
If you started out with full tank, i wouldn't worry about refilling so soon just because of generator. You should still have plenty left.