It is a cheap option (since 2001/2) that most people ordered with the tow package on the HDs. Just checked and is still $35. Listed on their BTO feature:
Originally Posted by 1stDeuce
TRAILERING WIRING PROVISIONS FOR CAMPER, FIFTH WHEEL AND GOOSENECK TRAILERS
The Silverado® 2500HD is born to tow. Whether for work or play, the trailering wiring provisions help ensure a hassle-free hookup to your camper, fifth wheel or gooseneck trailer. This feature includes an additional 7-way wiring harness routed to the front of the pickup box for easy connection to front-mounted trailer wiring. The wiring provisions require heavy-duty trailering equipment.
• 2010 Jeep Wrangler Sport (JK), 6spd, 3.73, 3x12.50-17 BFG KM2s on 17x9 ProComp 7069s, Flat Flares, Stock Suspension, Shrockworks Stubby Winch Bumper w/Milemarker HI9000 & Viking line, Cobra 18 w/Firestick II, Clarion & Boston Acoustic Audio