Ive been doing some more research.
Supposedly, wrangler tanks come in a 15 and 20 gallon variety, but that quantra link above only shows 15 gallon.
Chevy blazer tanks are like 28.5" wide, which might fit back there, and are 12.75 tall, which would hang down a few inches, but not too bad, and they are 25 gallon tanks.
A fullsize Bronco tank is like 33" wide, by 18 long an 13 tall. I dont see a reason you could not turn the tank 90 degrees, and fill from the rear bumper. Both tanks would have internal pumps which have pros and cons.
Can anyone think why I could not turn the tank 90 degrees and fill from the rear? Having had my Bronco on its lid, I believe there is a rollover valve in there (or I have a good cap I guess):