I don't know wich is the A-10's APUs fuel consumption, but in most of the airplanes APU is around 200lbs per hour.
Respecting the fuel gauge, I think it must work with the DC power system. Must see the wiring manual to affirm, but that wolud be the logical thing. To sum up, when the airplane is UNpowered, the fuel gauge should be off. When you set batteries, it should mark the QTY. Maybe you can test it without batteries because it's connected to the DC HOT bus.