I have had a couple do that and I was lucky, the filler hole was somewhat large...so I recruited my daughter ..who has some really skinny arms....(one more way she doesn't resemble me
) ....and with those skinny arms she was able to reach inside with a wrench...
One time I welded a cheap wrench to a long piece of 1/4" by 1" flat bar and bent it around to fish inside the tank and get on the nut...
You could try to pound an easy out into the fitting and hold it from turning from the outside while you tighten up the nut..
Or grab it down low below the external threads with some needle nose vise grips and hold it from the top like that...
Or try a butterfly impact and shock the nut down tight by short bursts on the trigger..
Ok that's all I got.
Hope it helps, mikey