I have a 41 gallon tank from Aluminum Tank Industries. I'm pleased with the quality and the price was good. It is gravity feed but I also installed an electric valve and indicator light. The gravity feed filler neck valve works fine; I just didn't trust it completely from the outset.
I reset my trip meter and use the MPG readout to calculate my fuel usage per fill up. I would prefer to have an in-cab gauge for the auxiliary, but I went on the cheap.
I agree with what FJ12 posted. Less stress and peace of mind. I also buy less expensive fuel; I calculate tank payback to be about three years.
I average 11.2 MPG towing which easily gives me 750 miles of range; more than I can do in a day. Most round trips I take, I can do on a single fill up, then hit Costco when I get home.