What would cause a gas gauge to read high? I've had it to the dealership 3 times and they've replaced the sending unit twice and the instrument cluster once. It will read anywhere from 1/4-3/4 high. Sometimes it will just drop off to zero.

 

Most stuff like this is a bad ground. Electrical problems are hard to trace. I would just add some solid grounds as a starting point. Just buy some wire or real ground straps and ground stuff. It may be a huge waste of time but it will be cheap and won't hurt anything. Ground the bed to the frame, the cab to the frame, the engine to the frame, the metal part of the dash to the frame, you get the idea. That has worked for us a couple times in the past.