We tow a 7200 dry weight camper - 2011 Outback 312BH to Maine from Vermont twice a year. There are some pretty good grades on the way there but nothing compared to what you'll see in the rockies. Its only a 4 and a half hour trip but I am usually wiped and ready for a beer after it.
Its stressful, even with a weight distribution hitch and sway bar. The wind is going to push you around and you will be running 50-55 on any kind of long grade.
With a half ton, I would shoot for a total weight under 6500 lbs for your kind of trip.
The lighter you go the better, itll be easier on you and on the truck.
MPG was roughly 6 maybe 7. About a 250 mile trip and we would make it about 3/4 of the way before filling up.