I tow with a diesel (see below) - but there are many users successfully towing heavy with the gas option; while they may be a bit slower it'll get 'r done (with higher engine revs).
Maybe a gear change could satisfy you, cheaper than a new truck. The gassers also avoid the periodic $400 oil and filter expenses.
If I had your setup my concern would be the pin weight. I'm gonna just make up numbers but my guess is your loaded 5th weighs in at 18K, meaning a 25% percent pin of 4500. My guess is a gas fueled F250 has a cargo allowance of 3500.
