If you are considering a 3/4 ton truck, you may find yourself limited to diesel
3/4 ton and up are designed for "work"...whether towing or hauling and you cannot beat a diesel for "work". While diesel may not get you that much more mpgs when towing, and they do get better mileage while towing than gas, you do get a lot more of what towing needs. Torque. Horsepower doesn't work, horsepower goes fast. Torque works and diesels have a lot more torque than horsepower. My 6 cylinder diesel is only 305 HP, but, it has over 800 ft/lbs of torque. Look at the OTR trucks...you won't find a gasser in the bunch, because diesels get better mileage (if only a mile or two towing), last longer (the diesel in my truck will outlive me - it's first overhaul isn't until 300,000 miles) and can tow like there is no tomorrow and do it all day long without breaking a sweat. Torque does "work" and my diesel has 800+ ft/lbs of torque, you won't find torque like that in any gasser, no matter what the size and as the size of a gas engine goes up (to get "torque"), the fuel efficiency goes down.
We just got back from almost 11,000 miles (two miles shy) of cross country traveling. I averaged 10.8 mpg towing not quite 12,000 lbs. and it effortlessly climbed every mountain pass we came upon and did it at my towing speed of 58-62 mph without downshifting more than one gear and it does it all at 1500 to 2000 rpm. With the built in exhaust brake, I actually had to "give it gas" to make it go downhill. My Jeep Commander, with the Hemi, would only get 10.5 mpg towing 6,000 lbs and on many passes I would be in third doing 45 mph and then stay in third and ride the brakes going down the other side. When I'm not towing, around town, my 3/4 ton diesel truck gets 17.4 mpg in mixed driving while a friend of mine down the road with a lighter F-250 gasser and is lucky to get 13 by his calculations. Yes, gas is cheaper, but, usually only by 10% and a diesel usually gets better mileage by ~25-30%. As for "diesel not being everywhere"; that isn't what I found in my travels this summer. While not every gas station sells diesel, I don't think there is anywhere that is "no diesel to be found" and I was in some "nowhere, usa" towns. I carried spare diesel in 5 gallon cans "just in case". I unloaded the unused, untouched cans when we got back to use in my tractor. In fact, in traveling, I found more stations with diesel than not...this is a big "country" and being in the country means tractors and work trucks...diesel is more available than you (and I originally) thought. While in a downtown metro area, diesel may be scarce (and that will change), you won't be doing that much towing downtown.
Get what you feel you need and want, but, don't discount the value of diesel, particularly when talking about a 5th wheel. IMHO, you are going to start seeing more diesel automobiles by the big three in the very near future...the engines are as clean burning (some even more so) than gas, the engines are cheaper to make (less moving parts and precision needed), you can put in a smaller diesel and get the same performance as a larger gas engine, they get better mileage and they last virtually forever; the engine will outlast the sheet-metal of the body on most vehicles and diesel is actually cheaper to refine. Resale value on a diesel truck is also much higher than the same truck with a gas engine. Why? Because people buy 3/4 and 1 ton trucks to work (unless you are an "Urban Cowboy") and diesels work, cheaply and efficiently, over the long run, and a diesel isn't even considered "broken in" until around 75,000 miles. I've got 27,000 miles on mine and the mileage is actually getting better; it is just starting to get "broken in"
My "high" for this trip was 12.8 mpg and that's towing almost 6 tons that is 8' wide and 12' 6" tall.
I've always been a "gas" person...I only towed bumper pulls that were relatively light or a utility trailer. Gas worked for those, but, when I went 5th wheel, I went diesel (almost had to with the size of the 5er) and, to be honest, if I ever go back to a lighter, smaller bumper pull, I'll keep my diesel, it is paid for and even towing my old 210WBS (I towed the 210WBS with the truck one summer just for kicks, felt kind of foolish at first, the truck is bigger than the 210WBS it was towing - I actually bought the diesel truck for work around the farm), it was much better at towing than my Hemi Jeep and I love my Jeep Commander. I'm a convert. I, literally, didn't know the 210WBS was back there. If you are going to stay with bumper pulls, a gasser is OK, but, if a longer, heavier bumper pull (30+ feet) or a 5th wheel of any size over 32' (and I'm one of those that don't believe in 1/2 ton towable 5ers) then a diesel may well be in your future and shouldn't be discounted out of hand for mileage and diesel fuel availability. Don't get me wrong, gas is fine, but, for "work", be it towing or hauling, IMHO, diesel wins hands down.
Addendum: I was just on another forum and looked around at some of the "gasser" mpg ratings that folks were saying they got; 5th wheel hauling with gas engines ranging from 6 to 8.1 liter gas engines the range was from 7 to 8.5 mpg. Larger gas engines to get the "umph" (torque) necessary for hauling larger loads spreads the mpg difference drastically. So yes, a 5.7 engine will get about the same mpg that the diesel gets when towing a load suitable for the 5.7 engine
, but, when the load gets heavier and a larger gas engine is required for the same performance, the spread between a larger gas engine, say 7.7 for an average, and a diesel's 10.5 mpg average, gets much larger.