Thanks for the info Workinrvers, coincidentally, I got a flyer from Camping World the other day "50% off vent covers and accessories". Lots of choices and princes at Search - vent covers - Camping World
Fan-tastic Ultrabreeze Vent Cover - White
has a convincing video comparing the air flow performance of different styles and manufacturers, but no data on strength. All of the vent covers use polyethylene plastic, polycarbonate would be a more durable material.
I wonder if spending $120 for two of these to protect $28 of existing vents makes any sense, maybe so if the RV is protected from the resulting water damage. If the new covers can't endure the hail, thats $120 a year plus water damage. Maybe a Rubbermade tote upside down with vent holes could be riigged to the roof.
Thanks for solving a mystery, I too lost a side marker lens and never figured out how it happened. It was at the same time the vent cover was damaged!