Go for the indoor storage; it is by far the best option.
We stored our units indoors over the winters for several years and then I decided to give covers a try because we were trying to sell our trailer at the time. It was OK, but here on the wet West coast I found there to be a lot of condensation at times. We then covered our new to us unit with an Adco cover last year and still had mildew on the roof and condensation issues inside the unit at times too. I finally found reasonably priced indoor storage and will be back to storing indoors this winter. It is cheap compared to having a leak over the winter.
