Make sure that you're really talking early September. Most campgrounds in Maine, Vermont and New Hampshire close down around Columbus Day (Oct 15th or there about).
The best campground in New England is Normandy Farms in Foxboro, MA, just south of Patriot Stadium and Boston. In southern Maine, try Libby's, literally on the coast - careful you don't back to far and fall into the ocean. In the Bar Harbour area, try Smuggler's Den Campground.
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