Start by removing the gauge to see if there is juice at the back of the terminals. If so, it sounds like a stuck float on the sending unit. To remove this,pull the sending unit out of the top of the tank. Should be a little round plate about 3" in diameter with 6 or 8 srews holding it down. With the unit body still grounded to the frame, turn the key on and move the float to see if the gauge resonds. It sounds like either the float is stuck in the up position, or, the sending unit itself has a bad coil, which, is internal and replaced as one part. All that unit is, is an electrical voltage device. When in one position, the juice is greater, and it sends the needle over to full. As the fuel level drops, so does the amount of juice being sent to the gauge, thus, the needle drops.
Sending units can be somewhat pricey, and you can fake it pretty easily, as long as the hour meter works. After 8 hours, calculate the amount of diesel you will burn and fill at the end of each 8 hour shift.
Hope this helps