No worries on the roundabouts (we call them rotaries). There are only a couple left. Just yield to the traffic IN the rotary.
Atlantic Oaks seems to be the highest recommended on the Cape and closest to the National Seashore. In addition, you have Bourne Scenic Park on the west end of the canal, Scusset Beach on the east end of the canal, Sweetwater Forest in Brewster and Nickerson SP in Brewster.
Nearby is
Normandy Farms in Foxboro, convenient to the commuter rail into Boston,
Boston Minuteman north on the Cape but still on the commuter rail.
Search the RV Parks section for 'Liberty Park' for info on the closest park to NYC, although I think a 40 ft 5th wheel would be a little tight.
