I have towed heavy with a 2015 3500 6.0 and with 2015 D/A and 2018 D/A. Towing about 30-31K combined up 6-7' mnt passes and living at 4700' elevation the 6.0 got 5-6 mpg towing that heavy. The Duramax trucks both older or newer are about 8-9 mpg. Obviously the Duramax is better towing experience, both up and down the hills (which you don't have). The mileage difference is not that great in my experience. Factor additional cost for Dmax and price increase for diesel over gas (at least here) it becomes a tougher decision. If you are pulling heavy a lot then the Dmax advantage really shows. If your pulling around 10K on flat ground, you have to knock down a lot of miles to get your money out of the Dmax.