My 2004 Roadtrek 190P has a slide down curtain to cover the rear windows. You lose 4 to six inches of valuable bed length if you use the slide down curtain.
Here's one way around that:
These came from a Dollar Store – they are folded in half with the edges lightly sewn and held in place over the windows with Velcro. I don't mind the reflective side both in and out - gives my LED lighting a boost