Thanks, kmac and Bob/Olallawa! I have learned something useful. I would imagine that the defroster would be the best way to direct cold air onto the windshield.
The only problem will be in persuading my poor DW that we need cold air, not warm, when we are driving in a snowstorm. (And we do quite a bit of snow travel --up into the Sierras. So this is not just a theoretical discussion.)
Another thing I learned by watching other folks in snowy weather -- when you park the truck next to the trailer, pull the wipers away from the windshield so they don't freeze to the glass.