It sounds like a great trip.
We have traveled the Northeast quite a bit and can offer some suggestions for campgrounds and interesting sights to see.
In Crossville, TN a great place to stay is Cumberland Mountain State Park.
If you plan to go to Niagara Falls and have passports, you can park on the US side and walk across the Rainbow Bridge to see the falls from Canada. It is a lot less hassle than trying to drive across with a camper. When in the Buffalo area, look for Ted's Hot Dogs - a local favorite.
If you are going to the Finger Lakes, a nice base of operations is Chute Park (Watkins Glen, NY)which is a township park located just across the highway from Seneca Lake. It's location lets you walk to many things in town. New York State has many wineries to tour. A nice rainy day option is the Curtis Museum located in Hammonsport, NY.
As you travel across New York State, Route 20 (east - west)is a great alternative to the toll road -NY State Thruway.
Lake George, NY is worth a stop. Take a boat tour as it is very scenic with lots of islands.
Maine has many places to visit. Arcadia National Park is a crown jewel. Allow a minimum of three days there. When in Maine,the local highways are very crowded in season so the Maine Turnpike is your best best when heading north or south.
Feel free to PM me is you have questions on specific areas.