Williams is a great little town.....old Route 66. And if you're staying in Williams, the Roadhouse is a great place for a steak! You order what ever kind of steak you want and they give it to you......the grills are on the customer side of the counter, along with all the spices, condiments, etc. You cook your own steak. Very clean and very nice atmosphere.....classic cars (he's always got 5 or 6 cars and trucks in there), etc. It's across the parking lot from the Harley Dealership.
There are a couple of boondocking campgrounds between Sedona and Flagstaf. Oak Creek Canyon would be ideal but not sure if there are any RV parks that I can recall.
Have you checked RV Park Review?