It will pull the mountains on this side of the Mississippi, you will spend time in 2nd to maintain speed on some of the steeper grades (like in WV). I think tow/haul does a decent job of helping slow you down too. Its no exhaust brake, but better than nothing.
I can't speak to how it would perform with the higher elevation and steep grades.
To be honest, if I had been able to drive it through the mountains with a load before I bought it, I would have gone Cummins right off the bat. This is my first non-Cummins HD truck. I wouldn't say I'm disappointed, but I was used to the low RPM driving of the diesel, so if you can deal with that and the mpg, I think you'll be ok.