Both US7 and US4 through Vermont are fine for any RV. Basically no road running east and west in Vermont is devoid of at lest some hills; that's just the way the land runs. Your proposed route is fine.
Another decent route is I-87 to exit 20, then NY 149 to US4, VT 22A north to US 7, and I-189 to I-89 south to Montpelier and US2 across from there to I-93 etc. US2 does run basically parallel (and often side by side) to I-89 for that stretch, but the interstate is easier going. US 302 through the White Mountains is a pretty road, but the Kancamagus Highway (NH 112) is even prettier in my opinion and is suitable for RVs despite being somewhat hilly and curvy.