Well Gang,
First off, I think this is actually pretty simple. First, if you have the land, square footage, etc. to install a drive under cover, and, you can afford one, no matter what type, and the permit process doesn't have you commit suicide, then you build one. Waaaaaay better than a "throw over cover" in every way.
But, if there's any speed bumps in any of the above, can't afford one, not enough land, don't own the land, permit process too much of a pain and restrictions, and more, and you still want to cover it, then a throw on might have to be your answer. Any cover is better than no cover when it comes to protection against the elements.
Now, one thing here, there is/was a difference of opinion as to whether a throw over cover will or will not convey "dirt" and dust etc. to the inside of the cover and therefore, will eventually damage the coach. Well, yes and no. You see, if you don't use a cover, your coach will get dirty, correct? So, now you wash your coach, and put on your brand new cover, however much a pain in the a.. it is.
Then, obviously if your coach is in a normal environment, that new cover is going to get dirty. Now here's where the "YES" (it will convey dirt to the inside of the cover and to the coach works. If you roll or, in any way, fold that cover up as you remove it, so that the inside comes in contact with the outside, guess what, you're transferring dirt from the outside to the inside. Now, when you "unroll" or, un fold that cover again to re-install it, you'll be DEPOSITING dirt/dust/debris/ etc. right under that cover, in between it and the surface of the coach.
But, here's where the "No" way comes into play. If you fold that cover, all the way from one end to the other, you're trapping the accumulated dirt, in between the two halves of the "top side". Now, you can roll it or fold it any way you want to finish securing it while it's not in use. Now, when you go to re-install it back on to the coach, you un roll or un fold it, to that point at which it has on big fold left. And that one will unfold in such a manor that it will keep the "already" accumulated dirt/debris/dust on the top side and not trap it in between the cover and the coach.
But, doing it in that particular manor, is seriously hard and just about no one does it that way. So, again, a drive under cover is superior but, the logistics are the factor. Good luck.
Scott