Massachusetts

Scusset Beach campground is on the ocean and the Canal.

+1 for Normandy Farms. Our go-to for a quick weekend getaway.

Bourne Scenic park is usually packed with seasonal types that live on the Cape and rent their home out for the summer.

No hook-ups? Sagamore Beach at the other end of the canal and Myles Standish SP in Plymouth.

You might have to plan for 3 different stays to keep from driving too much.
For visiting Boston, I would suggest staying at the Minute Man Campground.

Best CG in the area is Normandy Farms, in Foxboro. If you just need a launching pad for touring the area, it may be on the expensive side though.

KOA in Middleboro is a great central location for the areas you mention.

Bourne Scenic CG is right on the Cape Cod Canal and is also a great choice.