@afabulous I had some nice experiences some years ago at the northern unit of Kettle Moraine during late fall. It wasn't yet hunting season, so can't comment on how it gets that time of year.

It wasn't fully off-season yet, but I went up to Door County in mid-September and got a nonreservable site at Peninsula SP (usually booked way up during high season), and did some great day hiking at Newport SP (hike-in sites only, open all year 'round from what I can tell).

You also might want to look along the Ice Age Trail. I think many of those sites are year 'round, since people like to snowshoe/ski it and camp.