Boondock in Vermont ?

There is not a vast plethora of excellent options. There are some road-accessible dispersed camping sites available in the Green Mountain national forest, but they may not be very accessible to a fifth wheel.

For more urban areas, Wal-Mart in Williston permits overnighting (and I would guess probably also the one in Rutland, though I don't know for sure). There are also some on-street parallel parking spaces on Fayette Drive in South Burlington, next to the Lowes parking lot, where I frequently see RVs and tractor trailers parked (apparently overnight).

This is a listing from the official VT state parks web site of state parks with Columbus day weekend availability. They usually put out such a list for the major holiday weekends and typically update them every day or so. Whether the sites left can accommodate your rig would require further investigation.

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!

Go to a web site called "Freestays". You'll find a few places. I was born & raised in Vermont. Left there in 1964 when I went in the Air Force. It's one beautiful state, just don't want to live there.