stay the night at Cabelas.
Pretty sure it isn't allowed. I would call first. I would stay in Littleton at the Boston Minuteman campground.
8 hours from the state park to just west of Boston is most likely a pipe dream. Between construction, summer traffic and idiots it will most likely be about 10 hours minimum. On paper it should only take another 45 minutes to get to Manchester from Littleton BUT you can sit on Rt. 495 and/or Rt. 3 for literally hours at any time, with rush hour being the worst. And on rare occasions you can breeze right through but just plan accordingly. There are no rest areas with hook-ups and overnight stays are not allowed. And as stated above many are closed.