I am NOT a GM guy, but you might want to look at the GM with the 6.0. It is an older generation engine, but very reliable.
I don't think you can go wrong with the gas offerings of any of the manufacturers. The Ford 6.2 is an established engine with lots of history of being great. The Ram 6.4 is the newest engine on the block, but by all accounts it is doing very well.
My friend has a 2015 with the 6.4. He tows about 7000 lbs into the mountains of central and western PA without a problem. He got a gas engine because its his daily driver too. He loves his truck, his only complaint initially was the programming of the transmission. Going up hills it would lug in 4th, then drop into 2nd gear and kick the RPM's really high. I think he figured out how to drive it though, because he had no complaints by the end of towing season this year.
My father-in-law has a 2011 Chevy 2500 with the 6.0. It is his work truck, so it has a permanent work cap on it with tools inside. He towed a 5000 lbs camper, but just purchased an 8500 lbs trailer similar to mine. He hasn't towed in the mountains yet, but says his truck feels great hauling those weights (and high profile work cap).
Like I said, find the one you like that fits your butt right and your budget right. They are all great options!