Even September in Yellowstone has been "discovered" and you may have some difficulty finding campsites, especially those large enough for your rig. I'd suggest an alternative for that park: Grizzly RV Park in West Yellowstone. Hookups (remember, most NPS sites do not have them), clean & neat, and only a few blocks from the west gate of Yellowstone. We've used it as a base for visiting the park several times and have enjoyed it. Not as atmospheric as being inside the park, but many conveniences - and space for big rigs - that are lacking in park CGs. I strongly suggest reservations for anything in/around Yellowstone for August & September.