I've been towing with a 95 dually stick since it was new. I love the stick. Still going strong. Total weight of my rig is around 19,000lbs. Towing on flat ground it delivers a solid 12+mpg, 11 in heavy hills. The pump is turned up a bit. Other than that it's stock. Will pull 70mph up any hill. That being said occasional towing of a lighter trailer, the gas motor isn't a bad deal. Diesel fuel cost continue to climb so the millage difference isn't as great as it seems.
Doug