We enjoyed J&H RV park when we visited Flagstaff. Stayed twice in the past. Very nice people, clean park, well maintained.
However, I would suggest using Williams as your basecamp. I really like Circle of Pines KOA. It has large sites, easy hookups with many sites having multiple dump and water hookups making it easy for any rig. Clean, friendly, and just far enough off the highway as to keep the noise down. Lots of trees and a lovely indoor pool and spa. Williams is 15 minutes outside of Flagstaff, so shopping in Williams or Flagstaff is easy.
Happy Trails