I am surprised the 4.8 does as well as you say....
From your home to Colorado springs will not be an issue and from Flagstaff to home it will be down hill, sort of.
Once you get into the Rockies, it will be a pretty tough go with your truck. It will do it, but the world of 4000 rpms will be yours.
On the other hand, if you are relatively happy with your rig at home, I am not sure it would be worth buying a new TV for one trip. But, if this is to be the first of many such trips, I would dump that 4.8 in a heartbeat.
I bought a new Ram with the 5.7 hemi and it tows my 8000 pound TT with power to spare, even in the Ozark mountains....it would no doubt struggle in the Rockies. I get 19-21 mpg empty and 9.5-10.2 mpg towing. I bought a 3/4 ton, but I think a heavy half ton will work well for you. Some will say you will need a diesel, but that would be overkill, unless you are thinking about a bigger TT.
A 3/4 ton gives you bigger brakes, more stability, etc., but a properly equipped 1/2 ton with better gears, like 3.73 or 4.10 would do the job. I would look for a 5.7 hemi, 6.0 GM or a 5.4 ford
Good luck!!!