If I had plans to use a 5th wheel I would have gone with the Super Cab w/8' bed and Max Trailer tow that would have the 3.73 axles. And there is a little know option that few dealers keep in stock. It's a max payload package that use beefed up springs and 7 lug wheels and LT rated tires.
I use this combo for my delivery trucks. Only one has a 5.4 and the 5.0 I am trying my first Ecoboost in my truck.
On all my trucks I use the TIMBREN load boosters along with the Monroe Load Sencing Shocks. It's a gas shock with a coil over spring. Unloaded the truck rides normal. But when loaded it handles much better with less dip in the rear.