Get yer self a 120v 2000w electric heating unit for a hot water heater($10), snip a plug off of some electric crap you know you probably have laying around, wire it up to the heating unit and slip an exhaust gasket over the heater to keep it from falling in the tank. It sure helps to defunk a real nasty gas tank. It will still take mine 3 days of boiling and a gallon of marine clean. No wonder the bus didn't run
P.S. return heating element to home depot when done.