Are you going to want dual air conditioning?
The Ford wagon window vans have built in rear A/C and that is very helpful to the passengers in your humid area of the country! There are hundreds of E-350 vans out there, for sale. Manny are the cargo type, and will never come with rear A/C. It might be possible to install it, but expensive. Also the window vans come with good insulation too - both noise and heat insulation.
GM made a 8.1L gas engine for their 3500 vans for a few years. Not anymore. Depends on what you are towing, the 5.4L in the E-350 is a great engine, so is the 6.8L V10 - that is also used in the class C motorhomes with a 22,000 GCVWR.
The larger motorhome engines have much larger radiators, and a higher 362 HP engine, with a 26,000 (+/-) GCVWR. The E-350 vans only have a 2 valve per cylinder engine, smaller radiator, and only about a 300 HP engine rating.
Have fun camping!
Fred.