What length and weight 5th wheel? How many miles per year? Other uses for tow vehicle? Any west coast mountain passes?
Case in point the PO had a 8.0L gashog to tow the Arctic Fox 27' of mine. However the cost to buy diesel injectors or replace a diesel engine is extreme. I am happy to break double digit MPG with diesel. Gas is like 6 MPG.
Like said above, depends on the trailer, and your intended useage. If you're only towing an 8-10K pound fifth wheel 2 weeks of the year over moderate terrain, and the rest you daily drive it, then a 6.0l gas engine with the 6 speed auto would be more than enough. If you're towing 12-15K, plan on hitting actual mountain passes, and do it a few months out of the year, then a diesel would be reccomended. With the added cost to buy diesel fuel, extra cost to buy a diesel truck, maintence and repair cost's, the justification to own a dieses has gone way down, espescially for modern diesel's and all the added emission control's.
I'll argue that the tranny is equally, if not more, critical to the decision about what powertrain to use.
After moving away from a 4 speed slush box to a 5 speed manual (diesel), and having towed with a 6 speed manual (diesel) and 6 speed automatics (all gas), I'd never willingly go back to a 4 speed slush box again no matter what fuel the motor drinks. The 4 speed automatic is Ok on flat ground, but add grade and it just cannot compete with more gears.
And like others comment, knowing what trailer will go on the hook is most important. Not all 5'ers or TT's tow well.
More or less pick the tow vehicle after the RV is chosen. Otherwise you are working with the limits of the Tow Vehicle. Sometimes you are better off with a Medium Duty (4500, 5500, Freightliner) tow vehicle because a 44' RV Park Queen will put a real hurt on a 1 ton pickup no matter the config. Other than insurance the cost for a used MD truck can be lower than a high mile used diesel pickup.
Smaller RV is better - it's an RV! Unless you are full timing and even then the bigger units become real work to drive and park. You can visit more places with a 27' or less RV due to size limits esp. for the National Park campgrounds.
I've got a 32 foot(34' 7" overall length) bumper pull travel trailer. It can be difficult sometimes finding campgrounds to park in. Up to 25 foot is a walk in the park to find campgrounds that can fit you.
I went backwards for most people. Had bigger ones at first and now down to a single axle 18'. 1966 Shasta, Love it. I can fit that thing in my back pocket! haha. It was funny towing it with my hummer and pulled into a big gas station where a guy with a Tesla sedan stopped for some air in his 67 Shasta he just bought. Forgot my phone at home that day- wish I had pics of the 2 together. I joke with wife that once our boys are both moved out we will get an 8' harbor freight trailer to camp with.
my preference is diesel btw. even just to get groceries. Just get an older diesel that is low cost repair and good mileage. old 6.5 or 6.9, etc. Until Tesla releases their pickup- that might open up possibilities.
I always go for the diesel , it diesel has more power and durable, we are running a towing company and we are all using diesel trucks, we prefer ford truck cause ford is the world largest truck manufacturer, we are using for multiple services in 24-hour towing queens ny.