Just to clear this up, at least for Illinois. The poster who has the relative working at the scale house in Illinois is correct, private RV's don't have to weigh. It boils down to how they are registered.
Now if you see a big new FW pulling into an Illinois scale house look closely because he will probably have writing on the side of his dually. He will no doubt be "for hire" or in other words a commercial vehicle and delivering a new unit for one of the many factories located in northern Indiana.