....we stored our camper outside on the property this winter for the 1st time since new (2005). I built vent shields out of polystyrene XPS (30psi material), and capped it off with 1/2 inch outdoor (exterior grade) plywood (the plywood protected with an 8mil UV treated clear poly tarp "window"; the exterior plywood is heavily coated with marine varnish, 4 raw cut edges saturated with varnish, too). I taped the entire contraption together with special construction tape. I can stand on these vent shields (I'm 170 LBS), and they don't give at all:


Without these over our vents, the 3, 4, 5 foot snowstorms, and the dozen or two ice storms thrown in for good measure, and various and sundry branches that get blown onto the roof over winter (cracking and breaking during ice storms) would destroy completely both roof vents-- absolutely guaranteed. I made a custom woven polyethylene cover that just fits over the roof, and goes about 12 inches down side of camper; this is cinched down over the vent shields, using an elaborate latticework of elasticized tie-downs, and a few strategic applications of very robust outdoor tape (we often have winds here for days on end in the 60 to 70 MPH range on the mountain over winter).
We live in real snow country in eastern Quebec near the extreme north side of Vermont/New Hampshire border (we often have -30 to -40 below temps for many weeks or months on end), where it starts snowing by ~10th of October, and stops snowing in June, and as such, I've had to remove snow and large icebergs off the roof of camper and the 2 trucks at least 60 times this winter. We'll see if we store everything indoors at the marina next winter.
Anyhow, If this were me, I would custom-make a solution for my particular climate situation, and unique camper structure.
....I'm outa' here to shovel another foot of heavy damp snow off the roof of the camper (3rd time today),
Good luck,
Silver-