We bought our TT before we bought the "tow vehicle". The first few camping trips I towed using my '87 F350 work truck - 460cu gasser, 4bbl carb, creeper gear 4spd manual, 4.10 rear, dually flat bed. It got (still gets) 7.5 mpg - towing, empty, hauling, uphill or downhill. Actually, it is the truck in my signature picture.
I bought a 2000 F250 2wd 7.3 diesel automatic with 3.73 rear end. Stock, but Loud engine. Ran great. Got around 13mpg towing. Empty highway got almost 20mpg. Around town dropped to 9ish mpg. It was my first diesel, and I was impressed (if not shocked) at the cost of maintenance. Diesel in my area is 20 cents or more higher in price than regular gasoline. No more 5-7 quart oil changes, but 15 quarts of more expensive oil. Can't add just any radiator coolant, has to be special. Need to drain fuel filter every so many fill ups. Be ready for "bad" diesel, carry a spare Expensive fuel filter in the glove compartment in case you need to change it on the road. If you live in colder climates, older diesels need to be plugged in to warm up the engine block, or at a minimum allow it an extended warm up period first thing in the morning (neighbors love that). Just seemed like everything with the diesel was more expensive and special. Most local garages would not work on it, have to go to dealer or truck garage. But.... that diesel was a money saver over my beast of a gasser dually.
Traded that in, bought 2008 crewcab F250 5.4 gasser, auto, 3.73 rear. Back to 7qt oil changes. Quiet, smooth engine. Gets 9-10mpg towing, 10mpg around town, 15mpg empty highway. Cold starts not any different than warm starts.
A friend has a hunting cabin that is 2.5 hours away. He has a Chevy duramax w/ Allison, 2005ish truck. NOT towing, it costs him $15 more in fuel to go up there and back versus me in my gasser due to price diffence in fuel.
Another friend and his brother own an excavation company. They bought diesel pickup "boss" trucks a few years ago. They already were running a bunch of heavy duty diesel equipment and were used to working on diesel engines. After 2-3 years, they traded their pickups back in for gassers. As the friend said, when he "ran the numbers" for just basically running errands, the diesel pickups weren't cost effective.
So, after all that... If you are towing A LOT, diesel is the way to go. From what I've seen, diesel gets substantially better fuel mileage when towing vs gasser, but around town that fuel mileage advantage seems to disappear or if you are driving empty on the highway. If you are just doing weekend trips in the summer, I would say gasser. If you can afford it and just want it, then certainly get diesel! Diesel still does have GREAT trade-in value, I more than I expected when I traded in my 7.3.
