I-70 through Colorado

Well it is interstate so the passes are about as good as they get. Vail pass is a slog but certainly doable without any issues. Next is Eisenhower/Johnson Tunnels same as Vail, slog up and even more down to Georgetown but relatively easy. A couple shorter uphills after Idaho Springs then the descent down into Denver.

Nothing wrong with 15 down to 70 but you could save a lot of miles and time Taking US6 from Spanish Fork to Green River. I think it is along the lines of half the miles and a couple less hours of time. The road is good I personally would not consider it any drama but I guess that is really in the eye of the beholder. However if you do go that way you will miss driving across the San Rafeal Swell which is awesome scenery but if you really wanted to see that you could still take 6 to Price than 10 down to 70 and still save a lot of miles and time

It's an interstate, so the grades are not bad. We found that the lanes were kind of narrow in some areas, and there always seems to be construction going on.

But none of this is a deal-killer. 70 is still the fastest way across the state, as far as I know.

One thought, though -- many of the exits in mid-Colorado involve "roundabouts," those pesky traffic circles. Not a serious problem, but in a long rig like yours, you may need a little extra care when exiting, or else it could get exciting. ;)