We stay at Timberlane/South Philly KOA every time we go south. They are open year round but not quite below the "dewinterize" line.
The transient spots are in the front and pull through. Of course now that KOA has taken over, the rates have gone up. But it's about a 30 minute ride to Philly and not much traffic.
Going to Boston, take I-295 to I-195 to I-95 to Woodbridge then pick up the Garden State Parkway. If traffic is not too bad (weekday rush hours are miserable), then stay on the GSP to I-287 and across the Hudson on the Tappan Zee. ($14.75 for a motor home). To miss the GSP, from Woodbridge take the I-287 loop around to the west. Longer distance, shorter time.
After crossing the Hudson, you have a choice. You can take I-287 to I-95 (not recommended) or I-684 to I-84 to I-90. I-84 through Danbury/Watebury CT is miserable and Hartford can also be a problem.
If time isn't a factor, stay on I-87 north to pick up the Mass Pike (I-90) south of Albany.