In reply to the real dr gav:
We have had some great RV road trips in North America with our kids, but I would consider changing either your destination or time of year. Determined climbers may be able to seek out shade and/or climb early, but that won't help you fill the rest of the day with a toddler in temperatures likely to be reaching 100.
If you are not tied to school holidays, autumn is the best time to visit Utah and all but the highest parts of Colorado. As well as the temperatures being much more comfortable, everything will be much quieter, and you won't need to worry about campgrounds being full. Alternatively go high in the Rockies for cooler temperatures, but this is less child friendly.
I would thoroughly recommend a book called 'Fun Climbs Colorado: Best Family Climbing Vacations' by Sibylle Hechtel. It not only recommends child-friendly crags, but also good campsites and other activities in each area.
The Americans do seem to think very big on RVs, and we've always had to rent vehicles much bigger than we would normally need. However Cruise America do a 3 berth which looks more manageable. We were also happy with Moturist, though I think their smallest model is an overgenerous 4 berth.
To enjoy the wilderness experience from an RV look for National Park and Forest Service campgrounds, particularly if you're happy to do without hook-up.
Unmissable places? Too many to mention! But if it's not too hot, Bryce and Arches probably top the list.