We joined Harvest Hosts two years ago specifically because we wanted to visit Vermont and knew that we'd have a problem getting campsites without having reservations. It worked well and we liked it so much that we've renewed our membership and have used the locations around the country.
Here's a link to their map -
https://harvesthosts.com/maps You won't be able to get more information without joining but we stayed at four locations in Vermont and all were large enough to accommodate your fifthwheel. All were great places to stay. Enjoy your trip!