I haven't towed with the newest Chevy 6.0L gas, but I have towed 12,000 lbs worth of 37' 5th wheel with the Ram 6.4L Hemi with 4.10 gears and was very pleased.
It never got hot, never missed a beat, and got 7-9 mpg when towing (21,000 lbs combined weight) at 65 mph. It usually ran at 2200 RPM in 5th gear on the flats and used 4th for small hills and 2nd or 3rd for bigger grades. I know it sucks when tested on the "Ike Gauntlet", and I don't know what program that is that kicks in, but I never experienced that with mine. I towed all through the Canadian Rockies with it too.
I think you'd find the Chevy just as competent, just with less power. Independent tests by the magazines and websites that do such testing show the Chevy is down on power but otherwise would do a great job I'm sure.