I stayed there in 2010, on the way home from Yellowstone. We stayed at the KOA in Williams, right off the 40. I drove by the other KOA, on the way out to the park. There was nothing out there for shade, and it looked too new for the trees. I highly recommend the Williams KOA, a very nice park. We drove into the park, then took the shuttle buses out to the rims. They are free, and run every 15 minutes. We were there in July, so it was HOT, so the shade was important to us. If you are going in mild climate months, the newer park will work for you, and it's a lot closer to the park entrance. Have a great time there, as we did.
Steven