H E R E - Estes Park is a previous post on Estes Park. I'm up there frequently and this information is based on first hand experiences.
This is information on commercial campgrounds. There are USFS CG to the south of Estes Park, but none in the immediate EP area.
Also, you mentioned a fall trip. Do you have specific dates or months in mind? The season in Estes Park is relatively short. Closures begin in mid-September and in a few cases may go to late October.
There are also 3 CG inside RMNP, which are very popular, and for good reason. They put you in the park for the true Rocky experience. Reservations are a must, as sites go quickly. There is a 180 day window, so if this interests you, choose a date and figure out the 180 window date and make a reservation.