I had a bit of bad luck with my truck (2015 Sierra, 1/2 ton) so I had to rent a truck to pull my trailer home.
Ended up with a Chevy 3500 gas and was happy to try it out. 200kms to where the trailer was, averaged 15.8 l/100km, so a bit under 15 mpg through BC terrain. Not great, but not terrible. My truck (5.3L) would do a couple better.
Towing home, ~8,000 lbs, I must say I was underwhelmed. 30 l/100km (~8mpg) and struggled up some of the hills. Figure I gained maybe 5 km/h going up the hills. I expected better.
I'm going to see if I can rent a Ford 7.3 for comparison. Tough to find right now, but I hope to take a short trip, with the worst hill, with the same setup.