welcome to the forum Dave..
weight issues with a 3/4 ton truck always brings out opinions all over the place about how much load it can safely carrying.
GM gives your truck a 6200 rawr which is the lessor of the tires/wheels or rear suspension ratings. Basically the 2500 is the same as the 3500 SRW truck with its heavier rear spring packs....possible higher rated 18" or 20" tires and wheels.
5th wheel trailers place all the weight on the trucks rear axle so its to be addressed first and last. Many 2500 GM trucks rear axle may weigh in the 3000 lb range leaving it with around 3200 lbs for a in the bed payload.
Its best to weigh your trucks front and rear axles separately with all the gear/people/hitch...that way you for sure how much payload you can add in the bed.
The 3120 RL Montana website shows a roughly 12000 lb dry weight with around 4000 CCC = a 16000 lb GVWR trailer. By the time you load the trailer lightly it may be close to 13.5-14k lbs. Now add it all up and the trucks rear axle is at max or possible over on its rear suspension and 17" OEM tires and wheels.
I would look at trailer with a 13k-14k GVWR for a 3/4 ton HD truck.
