At the sender, look for the pink wire. Unplug it from the sender terminal. The gauge should read empty. Now touch/connect the pink wire to a good ground connection. The gauge should read full.
If it does, you have a bad sender or a bad ground, either the ground connection from the sender to the tank, or the connection from the system ground to the tank itself.
One other thing: Is there a switch to select which tank is connected to the gauge ( one gauge for both tanks)?