Another vote for the Grand Canyon Railroad RV Park. I just spent 3 weeks travelling in Utah, and stopped in Williams on the way back. Mostly I was in CGs on my trip, but I did stay in a couple of RV parks, which in that part of the world are usually large gravelly lots. I practically wanted to kiss the wide paved roads of the RV park. They also have some of the nicest bathrooms you will ever see. Another plus is that it is a very easy walk to the old town of Williams, which is a bit touristy, but nice. On the negative - internet was crap.