This evidently does vary by manufacturer. I remember that when we bought our 2500 Duramax, it was rated to tow slightly more than the comparable 3500. I just looked it up. These are specifically for
fifth wheel towing the 2500 Extended Cab Turbo Diesel is rated to tow 15,700 as Fifth wheel. The 3500 Extended Cab Turbo Diesel is rated to tow 15,200 as Fifth Wheel.
Here is my reference:
http://www.trailerlife.com/towratings/01towingguide.pdf