Hi,
Some dually's are not as good as the new ones.
The GVWR has improved over the years, and now a Dodge truck can carry as much as 6,600 pounds! Yet go a few years older, and they might only carry 4,000 pounds or so. That is a huge difference if your plan is to get a 4,000 pound camper with a slide out and options like air conditioner.
Get a towing guide for the perspective truck, and see what it's cargo rating is. Normally there should be a page for slide in campers.
Here is a towing guide for Fords.
http://www.fleet.ford.com/towing-guides/ I did a quick look at the 2014 Ford F-350 DRW, it has a 14,000 GVWR and up to 6,700 pound cargo rating for slide in campers. Reduced of course by passengers and accessories that are installed along with the camper!
You can use that link to get towing guides for 2004 to 2015 Ford trucks and all their cars as well.
Good luck!
Fred.