Rarely is 5 gallons of spare gasoline ever really needed. I'd switch to a 1.5 or 2 gallon plastic tank and just keep it inside the rear near the tailgate. I carried a 5 gallon jerry can for a couple years into some very remote areas including Death Valley and figured out I never needed it. I stopped carrying it around 2003 and never looked back. Not to mention that location helps keep the center-of-gravity lower than it would be by carrying gasoline containers higher up.
Guess I need to make a long trail run to confirm fuel usage. Lousy mileage from what I can tell. Usually 200+ miles in a day out here, including highway travel.