Tell me more about your plan. I live here and can help you more if I know what your preferences are.
1) Are you staying in Boston, or are you going in for day trips and want to come back to the RV at night? Are you looking for a campground with septic/water/electric? Are you a member of AAA? They have good data, tour books, etc.
2) Commuter rail is pretty good - there's a $10 weekend pass for as many trips as you want. Cash only, no credit/debit, unless there is a main station - otherwise pay the conductor, super easy. But they don't run every half hour so you need to plan. This is separate from the MBTA (Mass Bay Transportation Authority), which is the subways and buses. And "subways" are above ground outside of the inner city - trolleys that go underground when you get to downtown. You can pay cash above ground. You can also get a "Charlie Card" at various main hubs and then load money onto it on line as you go along. You cannot pay cash on an underground train. Lines are color coded and intersect on occasion so you can change lines.
3) I checked B's link - FYI "Littletown" is wrong, it's "Littleton" so verify the other things on the link. Salem is on their list and is a small city with its own attractions - and some narrow streets, so do some research.
4) It would help to know what you want to see/do in Boston or outside. There is a lot to do beyond the city limits too.
Message me if you want more specifics and after you've sorted out your plans a bit so I can be helpful.