A cover is a good idea, during the winter if for no other reason than it prevents the buildup of dirt that seems to accumulate in the cooler weather and shows up as black streaks. But be careful what you use:
1. DON'T USE POLY TARPS or anything like them that doesn't breath. They hold humidity and moisture in which will lead to mold.
2. Leave one or two windows
open just a crack to allow for ventilation.
3. I was told not to leave the cover on during the heat of summer.
A boat cover might be okay if it is breathable but be careful. I don't know if ventilation is much of a concern with boats as I see many owners having them completely covered in plastic wrap for the winter. Wouldn't do that to a trailer! For our 13 ft. Trillium
, I picked up an actual trailer cover that someone had returned to our RV dealer for a hundred bucks. It is actually designed for a tent trailer but covers down past the belly-band and so protects the most vulnerable areas.