I have a custom made 50 gal., not a transfer flow, Bed tank. Had to fit it between the bed locker roll up cover and the hitch. Gives me a total of 82 gallons, about 75 usable. For me it wasn't so much about saving money in fill ups, but I wasn't thrilled at the prospect of trying to wheel 67 feet of vehicle in and out of fuel stations every 150 miles. Now I can fill up after I've settled for the night. That right there makes it worth every penny I spent.
Could have gone with transfer flow, but with their shipping I got a better deal locally.