Your van will be safer without a cover. They trap dirt and when they flap in the wind it abrades the paint.
DO, however, cover all windows so that the sun doesn't attack the interior.
Now: 2022 Fully-custom buildout (Ford Transit EcoBoost AWD)
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)