As far as I can tell in Connecticut, gates close off the highway scales when they are not being used by DOT / State Police. When they are being used, it is mandatory that commercial trucks stop at the scales, and there is a line of trucks and a bunch of officers there. People would not be happy if a camper pulls in to get different weights.
My town uses a "transfer station" for garbage - garbage trucks and personal vehicles hauling garbage are weighed, they dump into a hopper, garbage gets hauled away to a trash-to-energy plant. At slow times, usually late afternoon, the place is almost empty. If you ask the scale operator, and are polite about it, they are perfectly happy to let anyone weigh pretty much anything at no charge.