Condensation may happen as a result of one of these two causes:

* the trailer is full of warm humid air (from a hazy humid summer day in the NE US) and then cools off at night to below the dewpoint of that air. A cover won't help with that at all.

* on a not-humid summer night in the NE US, the air in the trailer may not quite cool down to its dewpoint, but under a clear sky the trailer roof (especially if metal) cools further by radiation, causing condensation both above (not an issue) and below (inside the trailer). A cover may help with this - but only if it is thick enough to have a significant insulating effect.