My vote is for the diesel.
I have a 2007 Dodge 3500 SRW with a 6.7 Cummins and pull a 33' Mountaineer fifth wheel. Weight 10,575 pin 2,175. I checked the overhead monitor and it showed 14.5 mpg (I know its not totally accurate) driving 65 mph. I think with a gas engine, your mileage would be much worse especially with a bigger trailer. My truck has been deleted but otherwise it's not hard to maintain. I carry an extra fuel filter, but that's about it. 89,000 miles and it runs great. The built in exhaust brake is excellent pulling a trailer.