Easywind, what you're talking about is called a Fresnel Lens. Originally developed for large lighthouses, the principles are now well known and cheaply reproduced in plastic. You can buy fresnel sheets in many sizes, small or large, and it will stick to the glass without adhesive. I have an 8 x 12 on the pax door window to cover that last blind spot-- cost about $7. You could buy a good large one for your rear window for about $30 or so, I would guess. Check on Amazon, they have lots.
The image you see will be a little fuzzy, but will cover the full scope of the back of your MH. Remember, it's a mirror image 30-40 ft away. Detail is unimportant-- you want to see presence and motion, and this will do it for you.
However, as skip and zip said, as inexpensive as they are you might also consider a camera.
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element