I finally found it!! In my TM, the rear shell light shutoff is not controlled by the bathroom wall, but the bed platform. I had the bed pushed all the way into the TM (as when you're going to break down) and I shined a flashlight under the bed platform and viola! There was the silver button on top of the structure underneath the bed. When the bed is pulled out, there's a little white box looking thing (I couldn't get a picture of) on the underside of the bed near the front that will depress the silver button down. Mystery finally solved. Now, no clue how I could ever reach either part of this mechanism should I need to be able to do so. Looks like I'd have to take the bed apart.