The biggest issue I see with the Suburban and the towing results you experienced is that you've got a 5800lb vehicle (base curb weight) and you're loading it to 75% of its rated tow rating. In addition, let's be honest, a 3.42:1 axle ratio combined with a 16.25:1 power/weight ratio (33.25:1 with the trailer), is not an ideal setup for towing anything substantial. I wish GM would start adding the 3.73:1 axle to their Max Tow Packages in all truck based vehicles as standard. You're lucky you didn't end up with the 3.08:1 rear that comes standard. Your economy numbers would have been horrific.