Covers can do well to protect your RV under certain circumstances. It's a sacrificial layer but not without their cons. They are a pain to put on and worse to take off especially if they are wet. They can rub paint and contact points can scratch. And storing them while you use your RV is challenging, and again, worse when wet. I used one for a while until it tore and became too much of a hassle. Too many times I wanted to use my RV and there was snow on it or it was wet and then the problem eclipsed the advantages. Frankly I find it easier to wax my rv every 6 mos and maintain the roof. Plus where we live in the coastal South, covers do more to attract mildew, bugs and tree frogs under the cover than they do to protect the RV. To each his own. I bet there are more folks out there who tried covers and gave up than people that continually use them.
And they won't really protect from hail. They are a paper thin breathable material.