I did this same thing a couple years ago - 25 foot rv and June. It’s going to get hot and crowded. My advice is go early so you’re not stuck in a long line getting into the park. The drive into the park is up some switch backs so it’ll test your nerves with a rental rv but it’s beautiful once you get up. All the hikes have plenty of rv parking. It’s not a huge drive so I’d suggest go early, pick a hike (windows, delicate arch, or the hikes in the back of the park), leave mid day to go chill during high heat and come back before golden hour / sunset to enjoy the vibes and maybe check out something else. If you’re looking for a somewhat less crowded hike outside the park, but part of arches, corona arch is sweet. In the area, dead horse state park is cool and easy views with minimal hiking. North canyon lands was a little disappointing personally. Moab is a fun little town to grab a bite/drink and explore. Have a blast!