There are only a few mobile home parks in all of St Louis and St Charles counties, you would do better in Lincoln county or jefferson county (sorry, but there are far more there). Forget Chesterfield - there is one in Frontenac, but the city is constantly fighting with the state of Missouri trying to kick them out and the state sides with the mobile home park (they were there first). In St Charles, there is one on the outer rd, behind the banquet center that is on 5th street on the south side of 70, east of zumbehl. check there, they might have an opening. From 94 (north of 70 but headed toward 70), you can turn on the outer rd, and go west toward the river to get there.
I'm not sure of any on the 40/64 corridor. You might ask the municipalities, but most don't like to admit they have them. You might also check in with the companies that sell the mobile homes since they will know where the parks are or check the yellow pages.
You might be able to buy a lot that is further out (not in suburbia) but it would probably be south of 40/64 but we would have to search for one that the zonining would allow a mobile home and that would already have the utiltiies run to it.