The popular places fill up on the Big 3 weekends (Memorial Day, 4th of July and Labor Day), but you should have no issues getting a spot during the summer in most areas. Campgrounds on the ocean with water views would require reservations, the earlier you get on these the better. We do a lot of trips in the NE during the summer and we figure out where we're going a couple of weeks out and start calling around. Haven't been shut out yet.
If you're doing Cape Cod, Atlantic Oaks in Eastham and Sweetwater Forest in Brewster are nice. Normandy Farms in Foxboro is close to both Boston and the Cape, as well as Plymouth.
In Vermont, be sure to spend a few nights on Lake Champlain. Check the area on RV Park Reviews. We stayed at Champlain Adult Campground on Grand Isle. Beautiful spot.
We haven't been to the seacoast of Maine, but Acadia NP and the Bar Harbor area are very popular. We go to Sebago Lake in Maine. There are a couple of CGs on the lake.
Your route seems to be taking you through Niagara Falls. Nice area, most of the stuff to do is on the Canadian side. We stayed at Branches of Niagara. While in New York, be sure to hit the wine country in and around Watkins Glen or Ithaca. The Glen has 3 camping areas; Klute Park, a state park and a KOA south of town. We've done Klute and the KOA. Klute is in Watkins Glen proper on the lake and next to a Walmart. The KOA is south of the Glen. If you do the KOA, get a spot in the back, it's much quieter. You can hike the glen.
Rip Van Winkle CG near Woodstock gets good reviews. Just be careful of following the GPS to get there. There are some very steep grades in the Catskills, and you'd be better off staying on the bigger roads even if it adds distance. We also like Arrowhead RV Park and Marina right in the Mohawk River/Erie Canal in Schenectady.
If you want to see Boston, either Normandy Farms or Boston Minuteman park in Littleton are convenient to mass transit. Don't drive into Boston.
Feel free to PM me with specific questions.