I got it at Camping World, which is usually over-priced, but my wife found a great coupon. You can probably get it off Amazon more cheaply. And yes, it goes over the entire unit. It keeps out the rain completely, while still allowing great air circulation. The air vents are covered with a fine-mesh screen -- no bugs.
If you have no vent cover at all, you will probably need to create some kind of a closure so that your air conditioning and heating do not flow out of the opening -- the Maxxair does not have a closure. Maybe fabricate something out of a piece of light plywood, cut exactly to shape, with foam or reflectix on the edges, so that the board plugs up the opening from below and stays with a friction fit? It would not stay while in transit (too bumpy), but it would work when you are running the A/C.
The only downside of a Maxxair is that you have to go onto the roof of the trailer to install it. (I do not like ladders, especially at my age.) Installation was not difficult, if you are moderately handy -- if I recall correctly, I had to drill a couple of small holes in the side of the vent cover for a mounting bracket.