We just return from that trip with 10 year old GS. Sept will be less crowds than July. Saw a lot of animals in the Badlands, Big horned Sheep, Mtn Goats, Buffalo, prairie dogs. Stayed inside NP cg. Stayed in Wind Cave NP CG for 4 days visited cave as well as Jewell cave and Custer state park also worth the trip. Lots of elk, buffalo, and one moose. Devils tower was worth the stop as well as the CG there. Wall Drug was the same as it was 30 years ago you would't miss anything if you skip it. Mt Rushmore was nice. Save your parking pass and return at night especially if your a veteran. During the day at yellow stone it was crowded and could not find parking in popular spots, could stay inside the park for only one night the other 5 days stayed in west Yellowstone and drove into park. Latter in the after noon the crowds were less and saw 9 bears from 4 pm to 8 pm. Lots of buffalo, elk, etc also. Tetons were nice and Jackson Hole worth a day. No problems with CG's inside the NP. Flaming Gorge area was very nice and very remote. The water is very clean at the southern end and well worth the trip. Came back through Rocky Mtn NP and hiked to 12,700 ft to snow, it was difficult to breath for me but GS made it with ease. We had an accident in Ohio and ended up staying there for 5 days waiting for repairs to get done, but the Hocking Hills area was very interesting with caves and unique rock formations. Sept should be a nice time to go. Don't be afraid to stay at the NP or NFS cg's for a few days with out FHU's you can go 3 or 4 days without water and electric and it will add to your enjoyment of the NP's. Check your brakes to make sure calipers are not hanging up and change your brake fluid if you have not done so recently. The Mtns are steep and the down hills long. Have a good trip.