How deep is your interest in history? You've already decided there are places you wouldn't care about. Highlights for tourists can be a couple of days, but if really interested in history, Boston and the parts of New England easily reached from there can be worth years of exploration.
I'm going twice this Fall, allowing a week the first time, 2-4 days the second, but not taking the RV, it just doesn't work for the way I visit cities. Depends a lot on the city, either a couple of days is enough, or it leads to coming back again and again for a week at a time, a month at a time, and sometimes moving there for a couple of years to explore a place. Boston can be one of those "move there for a while" places if your U.S. history interest is high.