If you're on 64 heading west you'll come right to the Casino Queen RV Park on the Illinois side of the river.
http://www.casinoqueen.com/rv-park.aspxIt has an awesome view of the Arch at night.
When you leave you'll cross the river and immediately pick up I-70 west.
Edmund Babler State Park is also a very nice place to stay, depending on what you want to do in the St. Louis area. It has huge grassy lawn sites and is a beautiful park with walking trails, excellent biking and they used to have a pool - haven't been there in a while. It's conveniently located just off I-64 west about 30 miles from St. Louis downtown at the Arch. However, it's close to Forest Park with it's zoo, art museum and other activities. Leaving there it would be easy to get on I-70 west without the downtown traffic.
https://mostateparks.com/park/dr-edmund-babler-memorial-state-parkBoth of these will definitely need a reservation because they're very popular. Have a great trip!
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel