Guidelines as per GMC Sierra 1500 user Manual.
Tow/Haul Mode is designed to be most effective when the vehicle and trailer combined weight is at least 75% of the vehicle's Gross Combined Weight Rating (GCWR).
11500 lbs Oliver +Truck / 15000 lbs AT4 GCWR = 77 %
Looks like I am pushing over 77% since the weighing of 11500 lbs does not include all my gear and supplies.
Allen