[Supra_Freak_7m]

I would say you have a bed fuel level sender, it is corroded or the fuel level float is stuck at the top.

I have a similar problem with my car, If i put put a full tank of gas in it, the needle will go to 3/4 tank and then slowly sink down to a little over 1/4 tank. I have never taken it apart to pinpoint the cause. It could possibly help to go after the connecter for the sender with some CRC qd electrical connection cleaner. I do not know myself if the connector for the fuel pump and the sender are in the same harness or not.