I have heard of some people covering they trailers and others say do not cover so they can vent, they are meant to be outside I am talking about the tarp type cover's
I live in Oregon too, and covered our TT. I put it on at the end of Oct and take it off around the 1st of April. Wash it, wax it, cover it. In the spring it will look just like you put it away. We recently bought a new 5'er, now I have to get a new cover. Arrrrg.