I towed trailers in the 2500-3500 lb range with my 2008 HL AWD with factory tow pkg for 140,000 miles (out of 185,000 total miles on odometer). Both work and play. I kept it around 60-62 mph mostly, and 35-45 mph on grades. It did fine until near the end, when the trans temp light started coming on occasionally during tough tow climbs or extremely hot tow days.
I would recommend limiting frontal area as much as possible. My TT was 7' wide and 99" advertised height. This will be a bigger factor at highway speed than a few hundred extra pounds. If you don't have the tow package, get a tranny cooler installed.