I've had multiple RVs over the years and never had a cover because I have a pole barn. However, I'm tired of squirrels, birds and spring pollen making a mess on the top. So, I did some research and just ordered one of these off ebay for $185 and free shipping. All I want is something to keep the crap (no pun intended) off the top so it will not be so hard to clean in the spring. Says it is 4 layers and they shipped really fast. I should have it by the end of this week. I'm not sure how it will hold up if you don't have a shelter, but it might be worth you researching their product. I even had a couple of questions and they were johnny on the spot to answer.
4 Layer Covers 5th Wheel Travel Trailer RV motorhome camper Length 37' 41' | eBay