As people have said either gear ratio will easily handle a 10,000 lb RV. The Platinum trim level only came with 20" wheels and selecting 20" wheels automatically went to the 3.55 gears when I bought my 2016. I have Freund's who have the 3.31 gears behind their Powerstrokes and they experience a very significant fuel savings over what I get. Even though I see the value in a super cab long box over a crew cab short box my personal preference is still crew cab short box. I seldom have passengers but I like the ease of access to the back seat with the crew cab better as well as the look of the truck better. You do have to be mindful of what you're doing with a fifth wheel coupled to a shortbox unless you choose to get an automatic sliding hitch or a Sidewinder trailer hitch.
