I have a 6.2 and it tows my 10,000 GVWR 5th wheel just fine. In a normal year I'll put less than 2,000 miles on it so I couldn't justify buying a diesel just for the fuel mileage. I haven't been through the Rocky Mountains with it but I've been up some pretty steep hills. It will sometimes but not often drop down to 3rd gear on some of the big hills but I never lose speed. There's seems to be a big gap between 3rd and 4th gear so I think the 6.2 with a 10 speed would be fantastic for my use.
I'm pretty excited about the 7.3 but I might not ever own one. I'd like to have one but since I have a good truck there's no need to. If I did buy another truck down the road it probably would be a used 7.3 gas if it does get better fuel mileage than the 6.2. It would pull anything I would ever want to pull with ease and no more miles than I'd put on it buying a diesel wouldn't make sense. If I were going to put big miles on one then it would make more sense to buy the diesel.