Bangor to St. Johnsbury, VT

It's a perfectly fine route, no real problems at all. US2 is one of the key east/west routes through those parts, used by all sorts of vehicles. You could take it all the way and skip I-95 just as well. In either case, it'll most likely take a tiny bit longer than four hours with a sizable RV, maybe around 4.5 or 5 hours.

There are some curves and hills, of course, but nothing extremely steep or sharp or unsuitable. Any road in New England is likely to be curvier and hillier than most in Kansas!