Hi,
I think one of the BIG questions is do you need a dually, or will single rear wheel truck haul all the cargo and pin weight that your trailer will weigh? And since you don't have a fifth wheel yet, do you want to get a SRW truck then suddenly say "Sorry honey, you can not look at the Montana fifth wheels because we bought a SRW truck with a limited cargo rating, and must find a fifth wheel with less than 3,000 pounds 'loaded' pin weight and everything over 12,000 GVWR and 32' long is out of the question.." That might not go to well with my wife, yours might be more forgiving if you buy the wrong truck and suddenly need to change plans to a much shorter fifth wheel, or a longer travel trailer.
So get ready to do the trailer shopping too! Then stop by the local Dodge dealership, ask for a towing brochure. And once you have read it, you should understand everything you need to about GVWR, Front and rear Axle weight ratings, and the cargo capacity of various crewcab trucks.
You can 'get by' with a short bed and a slider hitch. However some slider hitches are only rated at 15,000 pounds or so. Check to make sure that is within the weight that your retirement trailer will be. I do like the Mega Cabs, they are very roomy!
But I would probably buy a regular cab long wheelbase dually truck and install a 'sleeper cab' (from a big rig) and a couch, wheel chair lift system, and probably 2-3 captains chairs. Then install a 6' long 33" tall toolbox body on the back. It would probably be 108" cab to axle, and the sleeper would be around 72" long, the hitch installed a little behind the rear axle (against all of the good advice offered here). This would be a 185" wheelbase cab and chassis truck, only about 20" longer than the wheelbase of a normal crewcab truck. Yes it would be a one-off truck and cab set up. But not to unlike the fifth wheel haulers sold on the Freightliner M2 chassis. They have something that looks like a class C in the front and fifth wheel bed in the back.
Good luck!
Fred.