As a local, do NOT and I repeat do NOT drive the RV in the city. If you are nervous around traffic and craziness, do not drive the city at all. If you do stay at Normandy Farms you are not far from Gillette Stadium and Patriot Place. However, you are 45 minutes outside the city. Minuteman Campground is in Littleton, not Lexington, also about 45 minutes away. That's if there's no traffic, so keep this in mind when determining commute times. The T (public transit) is a good way to see the city and it is a very walkable city. In 5 days you can hit the highlights-will take at least 7-10 when you start adding Concord, Lexington and some of the other historical sites outside the city, mostly because of driving time.
Lexington/Concord - 1 day if you enjoy biking, you can go along the bikeway between the sites - Lexington Common to Concord Bridge
The bike path stops in Bedford-there isn't a maintained link to Concord. A trip to Minuteman National Historic Park is well worth the time, though.