The gauge as another poster explained, is acuated by a magnet in the gauge face. As a float lever moves inside your horizontal tank, it sends a command to the dial guage and on through the two wires to your control panel lights. The guage at the tank gives you a back up reading to the panel reading. If your gauge hasn't, or doesn't move with propane use, take a hand held magnet and move the needle around to unstick it, that might be the issue with the dial guage at least.
The dial guage is an easy fix, if that turns out to be the issue. Otherwise, there might be a break in one of the two wires. But if the dial guage never functioned properly, that maybe the issue. Do let us know. Good luck.
One suggestion, call whoever your taking it to, and confirm they already have "on premise" the dial guage. Stealers are notorious for ordering parts after your unit is there, and think nothing of wasting your summer camping window. Just sayin'