What is your definition of a "nice" campground? Does that include hookups, or do you boondock too?
In terms of things to see if you like railroads the Kinzua State Park in Pennsylvania is pretty cool. It's an old railroad trestle they converted part of into a walking path after a tornado wrecked the center half of it. We went there this past spring and stayed at Tompkins Campground about 2 hours away in Tioga PA. They had full hookups there and I highly recommend that campground. It was very nice and very clean.
In VT there is the Green Mountain Flyer from Bellows Falls VT to Chester VT that is fairly picturesque I hear (I have never taken it, but I grew up just outside of BF). You can go to Allen Brothers in Westminster VT and get the best apple cider donuts. If you go early in the morning they will still be warm and crunchy... Yum!! I don't know of any campground personally to stay at in that area. Like I said I am from there, we are thinking about checking out Crown Point Campground in Wethersfield VT for a possible month long rental next year. My sister lives in that area, and she said she's heard it's fairly nice there, but again, has never stayed there, so take it with a grain of salt.
Smugglers Notch in VT is supposed to be beautiful, I know a lot of people from here in CT that frequent the area. Route 100 in VT is one of the top 10 roads in VT for beauty.
I have never "vacationed" in New Hampshire, sorta always felt weird to me since it's where I'm from!!
That said we have traveled the northern part of Maine a lot. Like people above said Route 16 is beautiful. We have stayed at only State Parks in Maine, so there's no amenities. But my all time favorite campground for atmosphere and nice wooded sites is Peaks-Kenney in Dover-Foxcroft. If you get to that area, go to the Nor'easter Restaurant. Best clam chowder... ever.
One of the most picturesque campgrounds we've been to is Lamoine just outside Acadia National Park. From there you can go to Bar Harbor and Acadia, both of which are amazing, there's an autoroad (no campers) up Cadillac Mountain in Acadia which is breathtaking. But a favorite of mine is Schoodic Point, which is actually on the mainland side of Acadia, and kind of off the beaten path.
If you go up Route 1 far enough, and like bridges and/or history, take a look at the Penobscot Narrows Observatory, it's in the pier of a 400' high bridge pier and has awesome views. We lost ourselves there for a day at the Bridge and then meandering around Fort Knox at the bottom of the bridge. It was really cool... Of course I'm an engineer, so I totally got to nerd out there for the day!!
Good luck!! It's a beautiful area over all, so you really can't go wrong. Most of the major roads would be more than able handle your rig.