If you know which wire it is back at the tank, you could cut the wire in a reasonably accessible place just ahead of the tank. To check gauge at that point, and the wiring from gauge to where you cut the wire, you can expect gauge to read one extreme with the wire grounded, the other extreme with the wire disconnected. (way full or way empty, can never remember which is which). Whatever, if the gauge moves from one extreme to the other (grounded vs. open), the gauge and the wiring leading to the tank (or wherever you make the cut) is good. At that point it's time to get serious about dropping the tank.
Don't assume it's a bad sending unit. Often as not, it will be about the ground wire from the sending unit to the chassis.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake