Easy answer: Weigh your truck fully loaded like you would be going down the road. Subtract that weight from your GVWR, the resulting amount is your payload. That will be the pin weight you'll be considering as your guide. Pin weight usually is 20% of total weight of a 5th wheel. 10,000 lb. fifth wheel usually has a pin weight around 2,000 lbs.
Rear Axle Rating trumps nothing, but some people use it as an ultimate amount. Some pay attention to GVWR, some don't. Lots of opinions.
I would think you could find a 30-34 ft. bunkhouse that will work with your truck without too much trouble.
