There is Archview RV Resort that I'd highly recommend you AVOID! I understand the owners were relatively new as of last May and they were jerks. They raised the prices on sites already reserved by my buddies, would not honor the price bargained for and where very jerky about it. I was unimpressed with their attitude when I spoke with them on the phone regarding my own reservations and ultimate cancellation (a seperate incident than the one listed above that involved my friends).
For free, I've often stayed at the Hole in the Rock rest area south of Moab. Its a rest area, but its clean and close to fun stuff.
I've been interested in this RV place for a couple of years now, but haven't spoken with them or stayed here: http://www.moabvalleyrv.com/
There is also a KOA in the area and several more RV campgrounds too. Now looks like I might be in the area, but I don't think it will be during May 11th-15th though, but trying to work it out.