Some of the campgrounds close in September, so that tells you that things slow down, a little. The one time we stayed in the park, we stayed at Bridge Bay and they closed a good part of the CG, this was in early September. Other times we've stayed outside the park in West Yellowstone at Baker's Hole, a FS CG 2 miles North of West Yellowstone. Another time we stayed near Henry's Lake at Red Rock RV Park but that was too far out for us. And yet other times we stayed at Flagg Ranch and Colter Bay, but both of these made for a long day, but we didn't mind the driving, but that's just us. I say to see the main highlights first time around, then hit the less traveled spots later or another trip. You can always, in the course of your drives, check out other CG's, then move there for a change.