It's often hard to recommend must-see things without some clue as to your interests.
The Lake Ontario State Parkway does allow RVs (with some height limits, though that's not likely a problem with a truck camper). So far as I know it's the only parkway in New York and Connecticut where RVs and non-commercial trailers are permitted.
That said, while it's a mildly scenic route (though without many substantial lake views), it's also in fairly poor repair with pretty rough pavement. NY18 is generally in better repair. West of the LOSP, NY 18 is also a generally scenic route with occasional glimpses of the lake. The Thruway is fast but boring, not especially scenic, and of course has tolls.
There is a neat looking little park and museum on the Thruway eastbound in Port Byron for the old Erie Canal lock 52; I think it's also accessible from the town without going on the Thruway. I haven't yet managed to stop on my way through when it's been open, but it looks like a fun place to stretch one's legs and maybe eat lunch.
Campgrounds should not be too full during the week, but weekends might be pretty busy.
If you like scenic driving and don't mind some fairly steep grades and fairly tight corners, a route that I often take when going between (northern) Vermont and western New York is NY365 from Verona, to NY 8 all the way across to Hague and then 9N north to the bridge to Vermont at Crown Point. This is a fairly scenic drive, without much at all in the way of development, and 8 in particular towards its eastern end has some pretty steep grades with pretty sharp corners. I don't mind taking my class C over it, but some would be understandably uncomfortable doing that sort of thing.
In Vermont, taking 17 to 22A north to US 7 north to I-89 S (via the short I-189 spur) and then on to points east is probably the best. VT 17 between 116 and 100 goes over the Appalachian Gap and is very steep with very sharp corners; though the view from the top is pretty nice. I don't generally recommend that stretch for RV travel, though it's not impossible in decent weather (i.e. not winter).
The state parks and state forest around Groton have some nice hiking and multi-use trails through and between them. It's a pretty area.
Crossing over towards Maine, I'd probably take I-89 to Montpelier, then follow US 2 at least as far as St Johnsbury. Going across New Hampshire there are a number of fine options: US 2 is good, US302 is also good and a pretty road, and the Kancamagus Highway (NH 112 from Lincoln to Conway) delightful. The Kanc has a lot of pull-offs and hiking opportunities, ranging from brief strolls to day hikes or longer, as well as a few (primitive) campgrounds along it.
I must say I found it amusing when I read you planned to drive east from Cape Cod...obviously you meant west! If you return via I86, the Corning Glass Museum is well worth stopping at. Not quite so much on the way, but beautiful places to see, are Watkin's Glen and Letchworth State Park.