Our F350 came with the smaller fuel tank. When not towing, which is seldom, I still have over a 400 miles range. When towing a fiver that is close to 16,000 pounds my range is less than 300 miles, which is okay with me because we stop every 2 or 3 hours anyway.
I carry three 5-gallon fuel cans in my truck tool box for emergencies, just in case we miss a fuel stop or end up in an area where fuel is hard to find.
I have never bought into the theory of having a large OEM fuel capacity plus adding an auxiliary tank. I know some folks that are hauling over 100 gallons of fuel. They think they are saving money because they can shop around and save .10 a gallon when filling up. Problem is they are lugging around about 500 - 600 pounds of extra weight they don't need to, which is probably taking care of all that savings.
Even if you do save a couple of dollars once and awhile by installing an auxiliary tank, it would probably take you 15 or 20 years to recover the cost of the install. I guess if you can get behind the wheel and drive non-stop for 12 or 14 hours then you may want to haul all that fuel.