I only haul a 3200 lb Truck Camper on mine, but also a 3500 lbs utility trailer some of the time. We did the Rockies several times, and I've been running the 6.0 GM w/3:73 engine for 214,000 miles with no issues. Just upgrades to the 2015 with 6.0 and 4:10 and like it better now when it's hauling. I actually get 3 mpg better mileage empty with the new truck and 4:10 six-speed, than with the 3:73 four speed ... and a slight bit better hauling, maybe one or two depending on climbing or not.
It's design might be a bit older, but it's truly a work-horse - and we chose it again over any other options. It hauls our weight fine up the steep grades, but yes, higher RPM, but that's how it's built. Fine power-train.
