fifth wheels
A 36' 'full time' suitable 5th wheel may be a bit heavy for a 2500HD. You'll need to take the pin weight of the trailer (can estimate it as 10 to 20% of the Gross trailer weight), add it to the hitch weight, and make sure that will all of your gear/passangers/fuel/etc in the truck that adding in pin weight and hitch weight doesn't exceed either the Gross weight of the truck or the maxium rear axel weight. It is less likely to be a problem, but you still need to check that the Gross Weight of the trailer and the loaded weight of the truck do not exceed the Maxium Combined weight rating.
Note that the 'maxium towing weight' claimed by the dealer is a load of crock. It assumes a stripped truck (no accessories, no passangers, nothing carried in it), a 150 pound driver and a gallon of gas. It is always at LEAST a 1000 pounds high and usually more. Furthermore, it generally ignores the effect of the trailer and hitch on the truck limits. Its purpose is to sell trucks, not keep you out of the hospital, the garage or court...
Be aware the the shortbed trucks can have problems towing 5th wheels. You will probably need an extended pin box, a sliding hitch, or both, to reach your tightest turning radius. Also, there are many people out there who state that long trailers and short tow vehicles are unstable. There are 'rules of thumb' about relative lengths, but I don't know them...