I spoke too soon. We camped with friends at Laurel Hill State Park in Somerset County. There were plenty of unused campsites beginning last Tuesday, and even Friday night campsites near us were empty, but Saturday morning, more campers came in filling some sites.
I don’t know if campers canceled because of all the thunderstorms and heavy rains that arrived toward dusk and lingered all night. Anyway, not all campgrounds are filled between the Fourth of July and Labor Day.