We have a 2012 GMC D/A crewcab combo with a short bed 4X4 and just finished a trip of about 600 miles, about 350 towing a FW, Jayco HT 26.5 RLS and a 19' Crestliner Superhawk in the mountains here in Wyoming up to Grand Teton. Total overall mileage, hand-calculated was 13.8. On the highway driving around 70 unloaded we get between 18 and 19 MPG. One thing with the D/A combo is that I never have to worry whether I have enough power to pull the load. Gas engines will inherently get a little better mileage unloaded and less when loaded. I think it's up to you whether you would like a diesel since it takes a lot of towing and unloaded highway miles to "pay" for your diesel premium of around $8,000. When I traded out of my old 2003 diesel I toyed with the idea of buying gas, but decided that I would stick with diesel due to the size and weight of my FW and boat. My FW is a light one, probably 8,500 loaded and the boat is around 3,500 for a total towed weight os about 12,000. Certainly nothing a good gas engine can't handle, but at what price for fuel and wear and tear on the engine? I don't know the answers and don't research this situation extensively, but I don't plan on getting rid of my GMC until I get rid of my FW.
