All the bed rails I've seen mount sideways. I think someone mentioned a 4way pivot hitch and they also pivot side to side. I started with a 14k single pivot hitch and soon had to switch to a 4way 15k cuz the single pivot bound up on really uneven ground and I couldn't get unhitched.
I've built 3 rv beds and about to build another and I put the hitch 3" ahead of the axle. When I started rving in the early 90s dealers included a single pivot hitch, installed, as part of a new 5er deal but I doubt if they do that anymore.
I bought my 32' 15k gvw 5er used from a friend and he already put on a 5th Airborn pin box on the 5er and it help a lot. None of my previous 5er weighed over 10k and no special hitch was needed for them.
I think your dually is still rated 12500 towing if you have the 4.10 but your payload goes up higher than my 3/4T did, so around 30-32', depending on how heavy it's built, will probably be your limit for the gvw.
Keep in mind, passengers and cargo in the truck are included in the towing capacity of the truck so estimate that weight and add it to the gvw of the 5er you pick, to avoid going over the trucks towing capacity. Craig
