Back in the day when I parked my TT on our property for over 15 years in that I covered it probably half of the winters. To be honest, covering it was a pain in the neck. It was a lot of work: I would duck tape pads to the corners because the fabric will tear over time; when I would take it off in the spring there would be crud/mildew on the TT exterior. The alternative was to stop covering it. Not covering worked out better for me in the long run.
We are blessed to have covered storage for our TT in a local storage yard. The expense is very worth to us.