you won't set speed records going uphill but you should be ok.
I have ram 1500 2015 with the 8 speed/3.21 transmission quad cab 4x4 and just got back from a Colorado trip. it definitely pulled better than my ram 1500 2002 with the 5.9 with 4 speed w overdrive (5th gear). not sure what the axle ratio was. pulled same 6500 lbs. loaded 26ft trailer. used tow/haul on 2015 which on the 8 speed knocked it down a couple gears. on the 2002 took one gear down (canceled overdrive).
the 2015 got 12.5 mpg compared to the 2002 which got 8.5 on the exact same trip. you will like the improved mpg's a lot, I did.
kept the speed to about 60 mph with an occasional 65.