July 4th weekend is, by far, the absolute busiest for northeast campgrounds. One that we typically stay at often has just a handful of folks during the week, is 3/4 full on normal weekends, and books solid for July 4th the day then open for the year. Most campgrounds (at least private ones) will have a 3-night minimum for holiday weekends. Fridays headed up 495/95 are already a headache, it being a holiday weekend this year will mean a nightmare for travel.
Your best bet would be to have reservations and get there earlier in the week, preferably by Wednesday afternoon since a lot of folks are going to be traveling on Thursday as well. And be prepared to pay for Thursday through Saturday nights...