Congrats. Looks like an awesome camper
My 2006 megacab diesel was/is notorious for not having any payload & I played around with different tires and wheels but feel it is now safe hauling my 2500lb dry Lance 830.
In the winter camping when I don't tow anything I run 3800lb American Racing 17" wheels and 4000lb 295/70/17 Toyo Mt's.
In the summer when towing the 7,000lb boat I run the 19.5's.
For your rig you need to weight the set-up and see what the weights are. Have fun in your new camper and make sure your tires are pumped to the max psi listed on them which should be 80psi.