A new gasser from any of the brands is going to get less than 10 miles to the gallon pulling a decent size trailer anywhere in the mountains which is where your headed I'm guessing. Diesels use torque not horsepower, obviously why they last longer because they use lower rpms. Maintenance is way more expensive on a diesel, whether you are leaving emissions intact on a diesel is the question. New diesels from 2008-present will get **** fuel mileage until they are deleted, I have a 14 megacab with a Cummins with 37's and pull a 32ft gooseneck that I get 12-15mpg, not pulling I get between 16-18mpg. Between the the lift, wider tires, wider footprint, exhaust brake, sway control, and a bunch of other built in features I feel completely in control no matter what kind of driving conditions I am in and that means a lot to me. Lots of people will tell you to keep your truck stock for the warranty, WORST advice ever!!! I have had 5 different diesels 2 duramaxes and 3 Cummins for a total of 500,000 miles and have never had a $9000 repair bill, never had any problems with any of them but this was because I removed the stock emissions equipment, that is what causes these trucks to need a warranty. Diesels do require more attention then a gasser, no you can't just jump in them, start and go. As far as warming them up they need more than 2-3 minutes in the winter, that's the problem with the new diesel trucks with emissions equipment, they recycle all the exhaust back into the motor as they are warmed up causing soot issues, which in turns clogs the egr...turbo...DPF....etc...etc.. Yes the dealer will warranty it all because they have to but after your warranty is up you will be paying lots of money for these repairs. The high idle comes on right away to warm the truck up faster to try to prevent these problems, is it good for your motor, hell no! With emissions equipment off you can allow the truck to idle as long as it needs to warm up properly, get way better fuel economy and avoid the dealership for warranty work completely. I deleted both of my brand new mega cabs with less than 1000 miles on them, have never had any problems with them but I take care of them - proper warm ups, synthetic oils, diesel treatment in every tank of fuel, religious service intervals, and I drive like I am white. I also prefer the way a diesel pulls/drives, I don't like the fact that gassers with or without the cruise control on requires the transmission to downshift sometimes very violently. Like a few others have said set the cruise with a diesel at 80 and you won't have to touch it. As far as winter driving conditions, while pulling your trailer up an icy pass do you want your gasser to downshift and send you sideways into the ditch or pull the pass at 20 miles an hour so it won't down shift or would you like a diesel truck that is in control all the time? The way it sounds your going to keep the truck along time so spend the money on a quality diesel, delete the stock emissions BS take care of it and love your diesel. I am a Ram diesel guy but the 17 ford is going to be a beast, so buy one!
