Where to stay in St Louis MO

Just depends if you want to drive and pay for parking in the city. We stay at Casino Queen. The big rig sites are paved with gravel in between and there is a nearby train track that has some activity. Even though this is located in East St Louis, it feels very safe because of the casino security. They will pick you up/drop you off at the metro and that makes it easy to see the downtown sites. We visited the Arch and also took a tour of Busch Stadium which was very nice. We also visited the zoo but we drove there. Zoo has no entrance fee but there is a fee to park ($10 when we were there). We never felt unsafe using the metro during daylight hours and if you want to attend a baseball game, the casino has a free shuttle that picks up/drops off right at the stadium. We had a very enjoyable stay while in St. Louis.

Missouri has some nice state parks and babbler is among he best.

https://mostateparks.com/park/dr-edmund-babler-memorial-state-park

You have a few choices including Babler State Park outside the city.

We stayed at the Casino Queen in E. St. Louis, IL, a few years ago and it was good for a couple of nights to sightsee one full day. Right by the river and you can see the Arch from the campground. Shuttle to the train across the river - one stop and you can walk to the Arch, boat rides on the river, etc.

It's just a parking lot with hookups, no atmosphere but very convenient. Prices have gone way up since we stayed there, though.

While you are waiting search this Forum in the RV Parks section for `St. Louis' or `St Louis' or `Saint Louis' and read on.
You could also do a Google search for `St. Louis tourism' and read what the specialists recommend.