I can also endorse the I15 to US6 to I70 route. I completed that route in a 24' RV towing a 4 door Jeep. There was no real issues (other than a few hills). It is the quickest route and I wouldn't hesitate to take it. It was about a 5 hour trip.
The other route is to take I15 down to Scipio and turn off on US50 over to Salina where you'd pick up I70. While I've driven that route, it wasn't in an RV. This would probably be a flatter route--simply because you would be on interstate most of the way. While the direct route does have some scenery, this route definitely has a lot more geology. You go through the San Rafel Swell area and that is pretty. However, it's 70 miles longer--so, probably 1.5 hours longer.
We have a second house outside of Moab that we are just about ready to head out to for the spring (once the 15" of snow from last week melts!). Let me know if you have any questions about the Moab area.
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