Grew up in Boston, so I am prejudiced - it depends on when you are going. Anytime starting in May and you are good to go IMHO. GREAT city, great area - you could stay at one of our favorite campgrounds (you are traveling with silver, right?) in Salem, right in Salem Harbor, called Winter Island. Boston is a must see town.
Charleston is great as well. Of all the choices you need to make in your life, this will not be your hardest - you can't make a bad choice.
2008 Airstream 27FB International Ocean Breeze
2014 Ford F250 King Ranch 4X4 w/6.7L Diesel, Hensley Hitch