Gee, I wish I had that problem.
Kidding aside, more information is needed. It could be the sending unit, the float, the gauge, the relay, the fuse, or anything in between.
Check ohmmeter readings between the various components and ground. When the tractor is off does it still read FULL? Is it usually on E when off? What is the voltage from the sending unit? Does it change when the tank is at different fill levels?
Can't say I'm an expert on such things but list what you have tried and one of the knowledgeable folks on the forum may deduce a fix.