Please post pictures of how you covered up the window(s) on your trailer's outside door. As you know at night you can see right thru them when there is a light source behind it. I'm just looking for...
You want simple? You got it! This is what we did -- I put two little cup hooks into the wall above the door frame. I then made a double-thickness hefty bag, reinforced with duct tape on the edges, with zip tie rings in the top two corners. The rings then go over the cup hooks. It sounds like a joke, but it works very well and is very easy to remove and store each morning, and it has lasted for about 800 nights of camping:
It ain't pretty. But then again, it is so dark that we can't see it!