The weather won't hurt your van, but a cover will. They get dirt and grit under them and the wind grinds it into the finish. You are better off without a cover. Just use window covers so the sun doesn't damage the interior.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)