When I lived up north, I knew one or two people who covered their steel trailer and it seems to have created more problems then it solved. The covers would hold the condensation in rather then allowing it to evaporate and it seemed their trailers had more then the usual amount of rust. If you have an aluminum trailer then I see no purpose for a cover at all.
I would prefer a shelter for my trailer if that was possible. Down here you can have a car port built for ~$600 and it will hold your car/truck as well as your trailer.