I guess you have a suburban heater with the metal plug and anode rod attached? If so, Teflon tape is probably counter-productive as you need to have electrical contact between the plug and the tank for the anode to work properly.
At any rate, a long enough breaker bar will not infrequently work wonders where an impact wrench fails.
On Atwood heaters, with the nylon plug inconveniently located behind the gas piping, the only tools I can imagine working are a 15/16" offset box-end wrench (which is virtually impossible to find) or some sort of channel lock or vice grip pliers type tool (which work, but tend to leave the plug worse for the wear and tear). I think it's telling that Atwood sells the replacement plugs in packs of two.