The Nudists in Joyland.
Indeed.
First paragraphs:
Shelly Bowman had to save her mother from the nudists. One summer day Shelly’s father informed her that Margery Bowman, who previously had been best known for her rock collection and pot roasts, had run off to “some sex camp” on the shores of Lake Michigan in Bear County, near the town of Bear River.
John Counts On The Internet 