If you don't need full a full hookup, WI state parks are great and open year round. There are two near the Dells and one about 20 minutes south. Reservations can be made on Reserve America. Not sure about availability. Fall can be a very busy time in the state parks. One place we went to once that was OK was Country Roads RV Park. Not fancy but clean and friendly. Don't know how late in the year they are open. It doesn't say on their web site.
http://dnr.wi.gov/topic/parks/name/mirrorlake/http://dnr.wi.gov/topic/parks/name/rockyarbor/http://dnr.wi.gov/topic/parks/name/devilslake/http://www.4countryroads.com/