Suggestions for camping near St Louis

Another vote for Casino Queen. It is safe, convenient, and inexpensive. Since you are "going to a conference," the fact that it is more or less a glorified parking lot with hookups will not be a drawback. Do be aware, however, that last time I checked (about a month ago) they were planning on closing for the winter in October, so you would be well advised to call them just to make sure they will be open for your stay.