Drove my motorhome through Arches and Canyonlands, and 45 foot tour buses go through all the time. Problem comes when you want to stop. Depending on how busy, there may be too little space for an RV at a pull-off where you want to stop.
My most recent visit was early October. Early morning wasn't a problem, evening wasn't a prblem, but early to mid afternoon there were loops with all parking full, and cars spilling over into no-parking areas along the road. Several of the stops in Arches need 30 minutes to an hour to hike back in to see what you came to see, and hike back out. If you can't park, you are going to miss that group of formations.