Hi Donn0128 - I checked the specs on our 2001 Chevy Silverado 2500HD 6.6L Duramax Turbo-Diesel. This is what is shows:
Both the Vortec 8100 and Duramax 6600 Diesel V8 permit towing trailers up to 12,000 pounds; they provide a maximum 22,000-pound gross combined weight rating (GCWR).
With a fifth wheel or gooseneck hitch, trailering capability increases to an astounding 15,800-pound maximum. The Vortec 8100 provides its maximum ratings with either a 4.10:1 or 3.73:1 axle ratio; the Duramax 6600 offers a 3.73 axle ratio.
So we should be good up to 15,800.