My route of choice would probably be NY 18 to Rochester, then I-390 to I-490 to I-90, then NY 365 from Verona to NY 8 across the Adirondacks to NY 9N north, and across the bridge at Crown Point into Vermont.
In Vermont, 22A north to US 7 North to I-189 to I-89 to Montpelier, and thence US 2 to Bangor, and Alt 1 or whatever to Bar Harbor. Rerouting via the Kancamagus Highway (I-93S) is a good scenic option as well.
NY 18 is a nice two-lane road. It would probably be wisest to avoid the Lake Ontario State Parkway that has very rough pavement. (Unlike practically every other parkway in New York, the LOSP does not forbid RVs and non-commercial trailers, though there are some comparatively low clearances in the 11'+ range.) NY 8 and NY 365 do have some hills and curves generally, and more specifically NY 8 between Brandt Lake/I-87 and Hague/9N is pretty steep with pretty tight curves. I have no hesitation taking my motorhome over it, but some would probably find it pretty intimidating.
If you don't care to take NY 8, cutting across from Fonda to Saratoga Springs (via NY 30A to NY29) saves some time and milage and aggravation and toll charges.
US 4 across Vermont is another perfectly reasonable route. jkwilson's suggestion of stopping at Letchworth State Park if possible is excellent, too; it's one of the most beautiful state parks anywhere.