We are purchasing a 2007 GMC Sierra with duramax diesel. We have a 31 ft. Cougar travel trailer that weighs 6850 lbs. empty, 9600 max load. We were towing it with a 2002 Suburban (with difficulty) using a weight distribution hitch and anti sway bar. Should we or is it necessary to use the WD hitch with this new truck?

GM recommends the use of the WD hitch for any trailer over 5,000 pounds. You already have it, I would say use it. It will improve the handling of the rig. The diesel power and Allison (if auto) is the ticket for your trailer.