Ft. McCoy is near Sparta, WI. I have no experience with campgrounds in that area.
I can tell you, though, that many campgrounds in that area and further north turn off the water and close for the season at the end of September. Be sure to bring warm clothes, nights can get to 32 degrees ot even below.
The trees will be turning, so it is a beautiful time to be in Wisconsin. There are many fall festivals, so be sure to check events calendars. The town of Warrens is famous for its Cranberry festival. They have a huge craft sale, and offer tours of the cranberry bogs.