I just saw a thread on this subject in this forum, tried posting to it, only to learn about the 104 day rule
This is a link to the 'old' thread; Grand Circle Tour Campsite Reviews
So here is the response I would have posted, and by starting a new thread we can all share some information on Red Rock Country.
We had planned almost this exact same trip this past May (2012), and after making all our reservations had to cancel due to our daughter being diagnosed with breast cancer.
She is now almost done with chemo (all is going according to plan) so we are now rescheduling the trip for May of 2013.
We had reservations at many of the places mentioned in this thread (and will be making new ones when the booking calendars allow), and in addition were/are looking forward to seeing the Vermillion Cliffs (highly recommended by friends that have been there), putting in for The Wave lottery (that just looks too cool to not see in person), and doing an Antelope Canyon photographic tour.
Looking forward to learning more from others on this forum and then sharing our experiences at a later date.