Most parking lots along the trail are administered by Missouri DNR, but I believe the one in St. Charles at Frontier Park is one of the few not under DNR control, but administered by the city of St. Charles. Try contacting the city of St. Charles Park & Recreation Department at 636-949-3372.

I once parked there for a week, but that was for a weeklong ride put on by DNR and was a special event. Not sure what the rules are for just an individual parking overnight. Best just to contact the city to find out their rules.