Hi smilinjack,
Welcome to the Heartland Owners Forum. There's a great bunch of friendly and helpful people here who can help out when you have a question.
You might want to visit
Fifth Wheel Safe Towing for detailed information, but here's my 2 cents worth.
You need to be concerned with both the 5th wheel towing capacity, and the vertical load/payload. A relatively new single rear wheel diesel 3500/350 will typically (not all) have a 5th wheel towing capacity of around 15,000 - 16,000 lbs and a payload around 4,000 lbs. The dually version typically claims a few thousand additional towing capacity and 6000-7000 lbs payload.
If you're considering a 5th wheel with GVWR of 15,000 lbs, you should plan on the pin weight running around 3,000. Add to that the weight of passengers, pets, tools, gear, hitch, bed liner, bed cover and anything else in the truck. It's not hard to get up to 4,000 lbs.
Can you safely tow a 15,000 lb 5'er with a late model 3500/350 SRW? Yes. But if there's ANY possibility that you might want a toy hauler or other larger trailer in the future, where the GVWR could be 18,000 lbs and the pin weight could be 4,000 lbs, now's the time to look hard at the dually.