I have heard of some people covering they trailers and others say do not cover so they can vent, they are meant to be outside I am talking about the tarp type cover's
Your roof and the paint/decals will last longer if protected from UV rays. I don't recommend using a tarp as it won't breath and is not soft like covers designed to be used on campers. Many people think that a cover is to keep the rain off, however, UV light is the primary reason for a cover.