As others said above, when we were in storage here in high altitude Denver, the UV would eat the poly covers in a few months, and the wind would get the canvas ones. It seemed like a waste of effort and DW and I jumped at the chance to rent a garage. We've been storing there for 9 years.
The garage saves on UV fade and wear and tear, as well as it being super convenient (don't have to close curtains, cover tires, etc.) However it is a negative investment as we have sold two 5ers that we garaged and even though they sold fast, we didn't get enough over market to pay for the rent on the garage.
Also the full body paint and fiberglass roofs tend to hold up a whole lot better outside than the rubber roof fiberglass wall RV with decals. That is worth the investment in my mind over a cover or garage.