There’s no such thing as a single rear wheel ‘99 Dodge 3500.
From the factory,your right, but there are quite a few conversions out there, just as I considered an F-250 DRW when looking for my truck.More and more people are converting to DRW and from DRW for whatever reason..It is more common than known..
To the op..Yes,a '99 Dodge used to pull a fifth wheel would certainly be an option worth looking at for using it for a truck camper..It simply means he was able to put the weight of the fifth wheel in the bed of the truck somewhat like a truck camper..He may already had some mods on it for the fifth wheel..