Here's a suggestion you might try. It's cheap, works well, and if you don't like it, can get rid of it instantly. First, our story:
When we go in the camper at night, we like leaving the outside door open and the screen door shut. This works quite well, except at night, anyone can see inside. In order to provide privacy, and still have the screen shut and the outside door open, I came up with an idea.
My first 2 campers, I did his with the awning puller. My 3rd camper has an electric awning, so I don't have an awning puller now. But I used a simple dow rod with an eye in the end.
You simply put a couple open tea cup hoods (or if you want, use 3m Command Hoods) and place either the awning puller or a dow rod across the door. Then hang a curtain, a towel, or anything you want. In your case, just across he window.
Here's how we did ours. My mother sewed up a pillow case for us so the rod would go through like hanging a curtain on a rod. We wanted the top of the door open and the bottom open (our heads and our feet), but wanted the middle covered (private) ... in case we were walking across the door with ... well ... very little on! :o
So I measured the distance, she sewed it to length, and when we want the ventelation, we do this: (You could adapt the length for your window):
Out side of camper door, no curtain:

Inside of camper door, no curtain:

I keeps the dow rod here all the time. When we had an awning puller, I kept right inside the door too. This was the place for it. Just put up a simple tea-cup hook or a Command hook:


With the curtain hanging. Remember, this was just a pillow case we open up at the seam:

