A friend put 3 cameras on his big Newmar years ago. One, high up on the rear and one on each side, right behind the "step", looking back. Had a 10" screen in the cab of his M2. He can toggle between a single cam, two cams or all 3 at once. He said it made is situational awareness a lot better, especially if he needed to change lanes. Biggest help was being able to see the idiots that would tailgate him.
Now, my cousin and her husband, rehabbing a '64 GM bus into an RV, put in a 7-camera security system from Costco with a 2tb drive in it. High & low cams in the back, one on each side looking back, one inside looking forward and 2 other interior cams.
http://www.beginningfromthismorning.comLyle