We camped at Normandy Farms CG - very convenient to Plymouth and the Cape, nice CG, IF you get a regular pull-thru site in the shade. We were in the "safari" area - open plain, like a parking lot for RVs. Not so nice, though the bathrooms & other facilities were fine. In general, I'd recommend it highly. About 1/2 hour drive to a major metro stop.
For the public transportation, buy a "Charlie" pass. (The folks at this CG can probably tell you about it, and when we were there they provided directions, maps, and general info about the system.) The Charlie pass gives you access to the subway, buses, and even water taxi (which we used to cross Boston Harbor to visit Bunker Hill).
Whatever you do, DO NOT drive in downtown Boston. Those drivers are insane. Seriously. (Have been for many decades now.)
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2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375