Thanks for all of the advice on this thread. We ended up finding a nice spot Friday afternoon along the Temple Mountain Road. And somehow we lucked out and had it all to ourselves. Even though Goblin Valley was swarmed with people our camp spot was pleasant.
Just as an FYI to future people, the idea that San Rafael is not good for RV's is just not accurate (at least in that area) Even during the crazy Memorial Day weekend we saw places to set up on Sunday. Granted you are going to be close to people but on a holiday weekend that is pretty much standard.