We stayed in the Middleboro KOA a year ago.
We never took the train in. We couldn't understand the ticketing. The parking charge was either a phone app or a higher charge to your license plate. My wife wouldn't put the app on and we didn't want to charge the rental car.
Driving to the subway (in Quincy) was unpleasant but we managed. One of the parking garages was closed for rebuilding.
There's a very pleasant Starbucks in a mall on the way to Plymouth. There's also Edaville which is a cranberry bog with an amusement park. Plymouth has a rock that people come from miles around to see.
If you like American history, or the kids are taking it in school, there is an awful lot in Boston. Also a couple of universities that can be toured.
If you like streetcars and subways, Boston has more variety than almost any other city.