I've got a 91 gallon tank for diesel in the bed of my truck. Add that to the 36 gallons the truck came with and I can tow my 19,000 pound trailer over 800 miles before needing to refuel. The real benefit is I choose when to refuel and that is always without the trailer in tow.
Back in 2009 I was towing a 24ft boat with a Ford gas truck with the 7.5 liter gas engine. It really sucked towing the 6% grades going to the lake. Borrowed a friends Duramax and was able to tow at the speed limit with the cruise on. That was unheard of with my truck. I bought my own Duramax and haven't looked back.