My unfinished DR has sliding bus windows in the GN, and I can't leave them open when it rains.  I found this product online, and it looks like it will work, but I'm not sure if it will make a difference which end of the window it goes on.  My windows open on the end away from the nose of the trailer. https://www.makariosrv.com/products/MaxxAir-Window-%7B47%7D-Maxx-Louvered-Window-Vent.html  Does anyone have any other ideas of products that would serve the same function, but might perhaps be cheaper?   

That is great. I haven't ever seen a solution except for window awnings and those aren't cheap.

The same product is $36.99 here. http://www.adventurerv.net/

I have seen these on travel trailers and they work great.  Check your local RV dealerships to see what they would charge and compare.  I have the MaxxAire roof vent and I have never had any rain come in it.  They make a good product.  Also try Ebay.