Chicago to Milwaukee is about 90 miles. Chicago is a city of over 6 million people. If you go in rush hour, it is tough sledding. If traffic is low to moderate, it is a fast high speed ride.
Hotel rooms appear to be booked from Milwaukee outward as far as Madison. We booked campground space at the Kettle Moraine South campground 37 miles from Milwaukee. It cost us $40 for 3 nights plus we will have to buy a $30 state park pass. Still a good price and pretty close to the action... or far enough out if we want to escape the action.
If you want to check campground space go to
http://www.reserveamerica.com You can then drill down to the closest campground and check availability and reserve a site.
You can also try to find a new friend who will share space... but if you fail, you are sleeping on your sled.
Have a great party and a safe ride from Costa Rica.