I would check the wheelbase for the 3500 with the crew cab and long bed and compare it to the double cab with the short box. I would guess that the difference is about 24 inches. The other change will be with the turning radius which is probably 4 feet greater. As you are already going with a 2 foot wider dually the extra 2 feet in length and greater turning radius are part of the package.
At least with GM a 2500 diesel SRW truck can handle a 5th wheel up to 17,200 lbs. and I and a couple of friends have pulled trailers with a weight of over 13,000 lbs. all over the western USA.
The trick with the SRW truck is to get a dampened king pin box like the ones by Trailair and other companies. These greatly reduce how much trailer motion gets telegraphed to the truck cab and its inhabitants. This works better than DRW in terms of ride comfort.
More options with SRW in terms of cab and bed configurations.
