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.