Like the others, I'd go with I-70. Ten's of thousands of RV's do this route every year, as do many, many semi's. You won't set speed records heading up hills, and remember to use your engine as a brake heading down hill. I-70 through CO is one of the most scenic stretches of interstate anywhere in the country.
If you REALLY want to avoid the high passes, you could take I-25 north to Ft. Collins, US 287 to Laramie, west to exit 187 and south on WY 789/CO 13 to I-70 and west to GJ. This is waaaay out of the way, and you'll be driving first through the traffic of Fort Collins and second, on state highways from I-80 down to Rifle. The state highways aren't bad and don't have steep grades, but some people don't like travelling on such remote roads. And there are a variety of grades on I-80 through WY, just not as high as those in CO.
