Ravialdo,
It sounds like you know the area so I guess you'll just have to wait and see how your family does. Maybe let them look thorough the book I suggested and pick places that they would like to see (lots of great pictures in the book). That way, they own the decision. Hopefully, you will be pleasantly surprised.
I assume you know this, but unless you have hookups, camping in a RV isn't like staying in a hotel as you need to use resources somewhat sparingly. We are seldom in developed campgrounds, so showers tend to be navy type (get wet & turn off water, soap up, turn on water & rinse off) every other day. With our solar, battery bank, and low power draw lights (LED), we have pretty much unlimited power (especially in that country). But I doubt a rental would have the this well developed system. Waste water management also takes some thought. I suggest you check out SaniDumps to locate possible dump stations along your route. Actually, RVing is a lot like backpacking in many aspects except that the resources aren't quite so limited:)
BTW, the outfitter in Escalante is
Utah Canyons Outdoor Store. I believe they have jeep trips into the canyons which would relieve you from having to battle your way down those roads with a RV. Not really our style (we take our 4x4 truck), but for your circumstances, it might work out really well.
Good luck and Have Fun,
Steve