We always stay at Camper's Haven in Dennis, but no pets in season - we love it there - real beach/Cape Cod feel, read cozy (small sites). Also stayed at Atlantic Oaks, good central location, Wellfleet area (oyster festival 10/12-10/14) backs up to rail trail. If you want to go all the way out to P Town, stayed at Coastal Acres (has an alternative feel to it, check out their open season, I think it's short. Like to go to Nickerson to bike/swim, but don't really like the campground/facilities. Bourne Scenic campground is at the entrance to the Cape and is a very nice campground. We are always so busy seeing/doing that we spend very little time at the campsite.
New England Unit
Metropolitan NY Unit