Hi Scotteam1,
Welcome to the Heartland Owners Forum. There's lots of useful information here along with a great bunch of friendly and helpful people.
There are 2 components involved in matching truck and trailer. One is the 5th wheel towing capacity. A Duramax 2500HD has similar towing capacity to a 3500HD, so you're probably ok there. The other is payload. There a 2500 is likely to come up short.
An Elkridge 39MBHS has a GVWR of 15,500. For planning purposes, you can expect the pin weight to be about 20% of that, or 3,100 lbs. Add to that the weight of your hitch, bed liner, bed cover, tools, firewood, pets, passengers and anything else in the truck, and it'll be pretty easy to have a payload of 4,000 lbs. Check the label inside the driver's door frame of the truck to see what your truck's load capacity is. If less than 4,000 lbs, you'll be overloading the truck.