School's out. Lots of kids with cars. Probably not a good idea, unless there's safety in numbers. Scope it out and decide.
Or, there are 2 Walmarts in Flagstaff, if you can adjust your timing. The one at the east end of town is newer and we've stayed there before, no problems.
We've stayed at the one just off I-17 in town and it's a bit more run down. No problems staying there, either.
There's a campground near Williams, on 47 off 64 north called Kaibab Lake Campground, that's very nice.
You might find a place to boondock in Seligman just east of Kingman on the old Route 66.
There are a few options in the area.