Third week in September some of the CGs inside YNP will already be closed, though not all. (Fishing Bridge should be open, but Canyon, for example, will be closed.) If you need hookups, FB is your only option in the park, as others have mentioned. Otherwise, I'd recommend Grizzly RV Park in West Yellowstone, as it's only a few blocks from the park entrance, very nice & clean, more elbow room than Fishing Bridge has, and offers FHUs, laundry, and proximity to town should you want to eat out or shop. Also as others have said, get reservations NOW, especially for inside the park.
If we don't get a cold fall, that time of year is a nice one to be in Yellowstone.