If you want to travel the West you'll need to do some mountain (hill) :) driving. I'll bet that by the time he drives 100 miles in the West he'll feel like a pro! Millions of people do it.
As stated, I-70 is an interstate - multiple lanes. However, it goes through a high mountain pass not far west of Denver.
There are easier ways to get to Moab... still some hills/passes, but not as high and the hills shorter in miles. CO 50 to Grand Junction/I-70 then west to Moab.
Another is to take 160 (you will have Wolf Creek Pass but very doable with a RV and multi-lane), then 160 west through Pagosa Springs/Durango then 491 to 191 north to Moab. Coming from Florida take I-40 and north on I-25 in New Mexico/Colorado and pick up either 160 or 50.
Or.. from I-40, take 491 north in New Mexico to 191 to Moab.... OR...from I-40 in Arizona take 191 north to Moab.
Or... take I-10 from Florida to some of those routes. Lots of ways not as white-knuckled as I-70 just west of Denver.
Don't avoid two-lane highways. You'll find them easy to navigate and will have gorgeous scenery and neat places to stop.... but do your research. Get the 'Mountain Directory for Truckers and RVers' and it will give you a 'heads up' on what to expect and what to avoid.
You're not just going to Moab, are you? That's for another post!!! Lots to do in that whole area.