We've stayed at Wompatuck many times, as my wife has relatives on the South Shore. You can call (617) 895-8245 to talk to them, but Columbus Day weekend is the last you can stay in the park. We ran into the same thing a couple years ago and had to vacate. Monday, Oct. 11 should be the last day in 2021 that the campground will be open. Unless they've changed their rules since we stayed there a couple years ago, they'll be closed for business from that day and you won't be able to stay the night of the 11th and beyond, but you should be able to stay the night of the 10th to depart on the 11th.
Mike and Cindy
2016 Tiffin Allegro Open Road 34PA
2018 Chevy Malibu