Although I store my unit inside, my opinion would be to cover loosely but secured. Your goal should be to limit extended UV exposure and debris staining from leaves etc. I would not shrink-wrap it as this would prevent any condensation build up from evaporating. It's amazing what UV exposure does to vehicles, boats and campers when left in one spot for an extended period of time.
2014 Ram 2500HD CTD MegaCab-Big Horn Edition
2016 ST Reserve 26RB