NY 365 to NY 8 is the route I usually take. There are some decent curvy grades on the easternmost section (between I-87 and Hague mostly), but you presumably would not be taking that bit of road anyhow. The whole thing is entirely doable with an RV, or at least I have no compunctions about taking my class C over it.
I think the time would be about the same as taking the Interstates; at any rate it shouldn't be an enormous difference. I went to RIT (in Rochester) for college while home was in Vermont, so I've traveled over that road and some of the alternatives more than once or twice.
A better alternative than NY-67 is taking NY-30A at Fultonville/Fonda to NY29 to Saratoga Springs and then getting I-87. You cut off more of an hypotenuse around Albany/Schenectady/Troy that way. Downtown Saratoga Springs can be a tad congested at times, but not too bad and not for very long of a distance.