The Man In The White Van
"The Man in the White Van" is based on a true story about the first victim of Florida-based serial killer Billy Mansfield Jr., who preyed on women and girls from 1975 to 1980. The story follows Annie, a fourteen-year-old girl from Brooksville, FL, who has a tendency to exaggerate. The movie begins when Annie notices a white van stalking her through her neighborhood. When her parents don't believe her, things turn deadly right around Halloween.
As noted by Deadline, the film "will explore the origins of the 'white van' trope in popular culture, and chart the beginnings of the destruction fomented by real-world serial killer Billy Mansfield Jr."
The story appears to be inspired by real events more so than directly based on them. Annie seems to be a fictionalized version of Mansfield's first victim, Elaine Zeigler, who disappeared while camping with her parents near Brooksville just before New Year's Eve 1975 (via Murderpedia).