One of the 'tips' for cancling a hotel room in New York City is first to 'extend the stay' then to call back to cancel while you have more than 3 days before your newly scheduled arrival.
So basically if you have reservations on the 22nd to 27th, then you call and ask it be changed to 24th to 30th. Then call back and say you want to cancel the reservation with more than 48 hours notice.
Anyway it is a good reason to avoid reserving a space! I hardly ever make reservations, so I am never in a campground that is so crowded that I can not get in there.
Good Luck,
Fred.