Ram 6.4/4.10 dually vs 2016 Ford 6.2/3.73 dually

Just a thought, and feel free to call it a dumb one. If it's primarily used for your camper what about a F450 cab and chassis. You could then get the V10 and (I think) up to a 4:56 gear.
I have no idea about securing a slide-in on a plain chassis though.

On a side note, I spoke to a guy hauling a decent size 5er with a Ram 6.4 dually. He spoke very high of it. Granted this was in Florida and he was from Florida, so take it for what it's worth.