I had a 2014 2500 gasser. Beautiful truck. Handled great. I'm only pulling about 7000lbs of bumper pull, and needed to keep in in 4th at about 2800-3000 rpm on the highway in the flats. I was getting 7.5-8 mpg. Which was ok and expected. It was surprisingly quiet and towed fine like that. Mind you, that was flat land and crosswinds. Throw in hills and strong headwinds, and I didn't enjoy it as much. Lots of revving and mileage dropped further. Throw altitude in the mix and results were down from there.
I'd just bite the bullet and get a diesel from the get go for that weight/surface area. You'll save money in the long run as eventually, at that wight, you'll end up with a diesel anyway after having thrown some $$ at the gasser by way of depreciation or trade.
'16 2500HD Dmax