I keep my travel trailer covered 11 months of the year (whenever it is not in use) and you can see from this picture what a difference it makes in keeping the trailer clean. The tarp goes most of the way down the front and back but only a few inches down the sides. This allows the trailer to breathe while still eliminating any chance that moisture can seep into any of the seams on the trailer.
They say that you are supposed to inspect/repair the seams every year but I find it is just so much easier to cover the trailer. This also means that the seams will last much longer since they are rarely exposed to direct sun and rain.
Some websites advocate that you should only cover your RV with a "breathable" material. I completely disagree with that notion since such materials tend to be very thin and tend to leak after just a couple of years. By contrast, our 18 oz vinyl material will last for many years without any risk of leakage. I have had my trailer covered for 3 years now and the top of the trailer looks as good as new with no signs of mould or age.
You have invested thousands of dollars in a travel trailer or other RV. Why not spend $300 to protect this investment and eliminate any risk of rotting and mould due to leaks? If you are paying to have your trailer stored indoors, you can save a lot of money by storing it outside under the protection of a heavy duty tarp.