I don't know about KC, but you will want to stay at the KOA in Pacific, MO about 36 miles from the stadium in STL. Do not stay at Jellystone Park, the spots are not even close to level and the price, while cheaper than the KOA, are over priced for what you will be getting. There is a park across the river but it is not in a very nice place--think East STL.
If you can maneuver in tight spots, you might be able to spend 4 nights at the casino and use the MetroLink rail system to get you across the river and drop you very close to the stadium.
There is one RV park on North Jefferson in STL, but that puts you right in the middle of the slums, not very desirable location, but the park itself gets some high marks in the GS RV Travel Guide & Campground Directory.
When we go to STL we always stay at the KOA, they are friendly and very helpful and the park is clean and well kept up. PM me if you have other questions.
