You will probably have to drop the tank. I would invest in a twenty five foot spool of 14 gage wire and put an alligator clamp on each end. That way you can attach the wire at one end to the battery ground and use the other end to find a bad ground. This way you can test to see if your gas gauge works before you open the tank up, just attach the ground to the lead going to the tank. Disconnect the lead the gas gauge should read full, attach the ground wire to the battery it should read empty.
You can check the rheostat on the gas sending unit with your OHM setting on your multimeter. It should read 30 or 90 OHMs depending upon what year car you have. Floats don't normally leak but with Gas-O-hol who knows.