I can't imagine that making much difference, if you are parked on ground and not concrete and sheath your unit on the perimeter, then it would make some sense in keeping the ground moisture from the underside.
Some winter campers put skirting around the unit to help the underside of the camper hold a little bit of heat and even put a couple of trouble lights down there.
Cold is cold tho, and anyting you do to insulate your unit will help. putting the plastic shrink wrap or celotex etc on the inside of the windows helps if you has single pane windows.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME