When our daughter lived in Kansas City, we liked traveling the distance in 2 days and we always planned an over night at the St. Louis NE / I-270 KOA. We've stayed there several times, but this was just for an overnight and next morning we'd be on our way again.
If you are planning on site seeing the St. Louis area, then most likely you won't be spending much time at the campground anyway. You'll be using your camper for a bedroom and on-the-go while you are there. If that's the case, this this KOA isn't bad at all. Granted, it's a typical KOA, pull through sites, commercial, and is similar to a parking lot with trees, but it's clean, safe, and well maintained. Full hook-ups and a reasonable driving distance from down-town St.Louis.
3157 West Chain of Rocks Road
Granite City, IL 62040
Link here.There are also one state park in Illinois that is within a decent drive of downtown St. Louis:
Lewis and Clark State Memorial State Park.
CAMPING
The Lewis and Clark campground is a great place to spend a leisurely vacation in the outdoors. The 82 campsites, all with electrical hookups, lie along the lakeshore. Modern rest rooms and showers are available, as is a trailer dump station. Starting February 13, 2006 advance campsite reservations can be booked through the park reservation system. Half of the campsites are still available for self-registration on a first-come, first-serve basis.