Love your whole plan, sounds exactly like our vacations, go go go!
Williamsburg KOA is good, also American Heritage RV Park is good. Water Country USA in Williamsburg is excellent, as is Colonial Williamsburg, and Jamestown.
When you go to Maryland, you will enjoy the Chesapeake Bay Bridge, lunch at Kiptopeke State Park right after the Bridge is a good mid point and a really neat place, easy in and out.
In Maryland, you will love the Boardwalk in Ocean City, I know you have state park reservations, but your kids would love Frontiertown Campground in Ocean City, it's pricey but worth every penny.
Plenty to do in New Jersey/Cape May, but I would skip all of it and focus on Ocean City, Maryland and surroundings.
Philly is great, especially on your way to NYC (which is a great way to finish things out).
