A friend of mine took a different approach, since his rig had a window across the back in the bedroom. He mounted a cordless computer camera on the wall below the window-sill, looking upwards. Above the window, he installed a mirror about half the width of the window and angled at 45 degrees. He then uses his small laptop (no longer used for other tasks) on the top of the dash and tunes in to the camera signal.
Nice clear picture, a decent size, already mirror image (since it's looking at a mirror) and no weatherisation issues with the camera.
Frank and Eileen
No longer RVers or FR owners