My kids (now in late teens and early 20's...) love St. Louis and it is only 298 miles from Chicago!
We stay at the Casino Queen in East St. Louis. Campground is very safe and they will send a free van shuttle to take you to the train station. From there you can buy a day pass and hop on/off at all the attractions.
Phillies are in town over Memorial Day so you can catch a St. Louis Cards game in the new stadium. Ride up to the top of the arch is really cool. Also there is an Imax theater. You can also tour the Budweiser plant!
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles