I have to chime in as I have a relatively long and heavier TT being pulled with my 08 Yukon XL Denali. About 90K miles on the odometer. My 35 foot Keystone Bullet weighs in loaded at 7400 lbs. Only 3 small kids, wife and driver are considered cargo in the truck so we're just a bit under with payload.
First thing that I did with my Denali when I bought it last year was add a second aux trans cooler next to the small OEM cooler. This makes a huge difference in temps in hot weather and on inclines. We just returned from a 2,500 mile round trip to Disney World's Fort Wilderness. The truck pulled perfectly. Ambient day temps were in the mid 90's the entire trip from MI to FL and my transmission never saw temps higher than 202 degrees (and that was only after our longest and steepest climbs over 6-7 miles in southern KY). Average temps hung out around 175-180 degrees at 65 mph. The 6.2L powerplant mated to the 6 speed 6l80 performed flawless. Yes, it did rev into 3rd gear several times to maintain speed on some of the steep inclines, but that's good thing. The transmission pump increases output and pressures providing the necessary cooling response.
2nd, add stiffer sidewall tires like XL or even LT. Made my towing experience nicer. Just an FYI
3rd, upgrade the TV's brakes (pads and rotors). Performance based products like EBC can effectively change the way your TV safely slows down the load. Augment that with a quality brake controller (I have Tekonsha's Prodigy P3) and my truck can orchestrate braking maneuvers as if the trailer wasn't even attached to the hitch.
Speaking of hitch, the next thing required to properly trailer a tow-behind of this size is a premium weight distribution hitch. After a white knuckle experience with a $500 BLue Ox Sway pro hitch on a return camping trip that had 30mph crosswinds over 200 miles, I swore I would need the biggest f'ing heavy-duty truck I could get my hands on. Then I discovered Hensley and their Arrow hitch. This hitch alone has made my driving experience 1000% better. When you talk about 1/2 ton vehicles moving loads close to capacity, a hitch like a Hensley or Propride is worth it's weight in gold. Cresting 7% grades at 70mph and being boxed in by multiple semi's with heavy crosswinds, the Hensley Arrow allowed me to drive 1 handed as I relaxed on the down hill. Not one single white knuckle event over 2500 miles on our last trip.
As far as octane, there is a noticeable difference of power when my Denali runs 87 vs 93. Haven't experienced and engine knock but the power output is reduced. I see that especially when climbing hills. The truck seems to work a little harder when it's full of the lower octane gas. I know the owner's manual recommends premium fuel. Confirming this, the greatest hp and torque gains with tuners comes at the cost of premium fuel. Sucks on the wallet but the engine loves the octane.
Again, these GMT900's perform well when properly equipped, weighted and setup for the load they are trailering. I have about 6,000 miles on our rig this year alone and we still have 4 more trips planned. The truck pulls wonderfully, engine and transmission temps are well-behaved, and the Hensley allows my wife to drive some too!
