Be prepared for some really bad roads....
This and
Plymouth Mass! Do the "pilgrim thing", check out Plimouth Village and the Mayflower II (fully functioning replica of the original Mayflower). The ship is usually taken out for a sail one or more times a year, so try to make sure you visit when the ship is in dock.
Although the actual "rock" has been called the world's most disappointing tourist attraction.
On the route, Mr MAC has it correct, although to miss the worse part of I-81 (north of Harrisburg) there are 2 alternatives.
1. Take I-78 to I-287 around NYC metro area to the Tappan Zee Bridge, then I-684 to I-84.
2. Take I-80 to US209 up the Deleware Water Gap (pretty ride) to I-84.
For seeing Boston (don't even try to take the motor home in), you can base at Normandy Farms or Boston Minuteman in Littleton. For Plymouth, there's Miles Standish SP, but no hookups. Plymouth is 45 mins from Normandy Farms. The closest is Scusset Beach at the eastern end of the Cape Cod Canal.