I have a cover and have used it a couple of times, but my biggest concern with a cover is they are a pain to put on and take off. It's actually a dangerous thing to do because you have to get on the rood unless somebody has developed a better system that I don't know about.
The main thing that is going to harm your motorhome is sunlight. If you have a way to build a roof over the motorhome that protects it from sunlight, in my opinion that is just as a good as a cover if not better.