The best public camping is a half hour to the west in Kettle Moraine State Forest; (262/646-3025, open year round) it’s very popular and is often booked, so arrive/ reserve early. The nearest private campground is southwest of Milwaukee [1] along I-43 in Mukwonago at Country View Campground (S110 W26400 Craig Ave., 414/662-3654, $23–24).
Country View is open mid-April–mid-October and offers a pool, playground, hot showers, and supplies. RVs can camp at the Wisconsin State Fairgrounds, west along I-94 in West Allis, but it’s crowded and cacophonic with freeway traffic.
Links:
[1] http://www.moon.com/destinations/wisconsin/milwaukee