Greetings,
Google Wompatuck State Park in Hingham, MA. Private, spacious sites. About 35 minutes to Boston. There's also a high-speed boat shuttle in the harbor to Boston.
If you have a large vehicle -- DO NOT bring it into Boston proper if you value your coronary health. Unless you're familiar with the city you WILL get lost.
There is no logic whatsoever to the street layouts in the city and parking is very limited even with a small vehicle. Plenty of hop-on/hop-off shuttle buses, etc. within the city for the growing tourist trade. A good Rapid Transit System also, called the MBTA.
Try the Duck Tours with (WW2 DUKW Amphibious} Ducks. They tour the city and then splash into the Charles River for a little cruise before climbing back onto solid ground again for the return to base.