What sort of a rig are you driving?
From Niagra Falls area to Maine, I'd probably take NY18 to the Lake Ontario State Parkway to Rochester. At Rochester, 390 to I-90 (or 390 to I-490 to I-90 to save a bit in tolls).
Thruway to Verona, then 365 to 8 through the Adirondacks, to 9N and cross into Vermont via the crown point bridge. In Vermont, 17 to US 7 north to I-89 to Montpelier and then US 2 out towards Maine. I like the Kancamagus Highway (NH 112) through the White Mountains of New Hampshire (and it's perfectly reasonable for RV travel), though there are many pretty roads through that area.
8 through the Adirondacks, particularly towards the eastern end (between Hague and Brandt Lake), has some relatively steep winding portions. I went over it without great difficulty or drama in my 32' class C not long ago.
Unless you have something like a pickup camper, resist the temptation to take VT 17 over to VT 100. While it's a pretty drive, it's quite steep and has very sharp curves.
