I have a 2012 2500HD 6.0 3.73 Crew Cab. I pull equipment trailers with it so I don't have wind resistance an RV will, I pull 14K routinely and in a pinch, considerably more. I don't believe the "you need 4:10's over 10K" argument. I live at 5K feet. The truck pulls real well. I use the manual gear selection at high GVW and high alt. no problem other than poor mpg.
Pulling 14K in our terrain it goes down to 6 mpg. Empty I get around 14 mpg as long as I don't go over 70 mph. I have slightly oversized 18" tires on it.
I have 40K on the pickup now. I really like it. I use it anytime it makes sense to. Mine is a completely loaded LT. The backup camera is really nice. Zero warranty work on it. I change the oil and drive it. Its no Duramax, but it didn't cost as much either and the fuel is a $1.00 cheaper.