Since it is a private campground, you can make reservations anytime, although it is difficult to get them on the phone in the winter. Email (camp@riversidepark.net) might work better.
I made mine when I left the 2021
rally. Unless things change, it is scheduled for September 16 - 18th, with some folks coming early and/or staying late. I'm registered arriving on the 14th after coming from the Algonac
rally in MI the week before, and leaving on the 19th.