Interesting... I was just wondering the same thing the other day. My wife and I have friends in Milwaukee and used to camp out in their driveway. They sold their house, so now we are looking for something else.
There is a nice campground north of Racine, WI, which is about 20 miles from Milwaukee. I believe it is called Cliffside Park and is part of the Racine County Park system. There is also a Yogi Bear Jellystone campground in Caledonia, WI, just off I-90/94 north of Racine. The Milwaukee State Fairground also lists an RV park on the grounds in western Milwaukee, but I don't know anything about what it is like.