Route 9 is perfectly reasonable.
US 4 from Fair Haven, through Rutland and to White River Junction is also fine.
My choice would probably be 22A (crossing into Vermont at either Fair Haven or at the Crown Point Bridge, the latter case taking 17 to 22A of course) to 7 to I-189 near Burlington to I-89 to Montpelier, and then US2 across...or maybe US 302 across and then the Kancamagus Highway through the White Mountains. The Kanc does have some decent grades and curves, for sure, but it's a good road and a beautiful drive.
Two roads to definitely avoid with an RV are 17 between 116 and 100 (through the Appalachian Gap) and 108 between Jeffersonville and Stowe (through Smuggler's Notch). I have gone up and down one side of the Appalachian Gap with my motorhome, but I'm familiar with the road and knew what I was getting into--and still took it very carefully. I will not try to go across Smugglers Notch with my motorhome. Most every year a few semi trucks get stuck wedged between the rocks there, despite the very, very clear signs that they are not permitted on the section of road.
