Keep than cover really tight!!!
I was a fan of covers until one year my cover came loose ( on my previous 5'er) and I didn't notice it till spring. I agree a cover can really help with UV damage reduction and keep the trailer looking nicer, but if any of the cover is too loose fitting or is allowed to flap in the breeze, you will regret ever putting a cover on it. The edge of the rubber roof is rolled over the top edges, it the cover is allowed to flap against it, it will damage the rubber roof edges and you will see rub markings in the siding and front gel coat cap and vinyl decals ( not good). I no longer use a cover, since it would only take one good wind storm mishap and all the good a cover could have done is more than lost in damage by a cover coming loose for a length of time during during high to moderate winds.
Unless you can check the cover on a weekly or more frequent basis to ensure it is tight and hasn't come loose, I would highly recommend not putting a cover on.