Suppose I removed the rear windows in the 4104, which I can't see out of when I am driving anyway. In place of the window could be a weathertight door, hinged at the top. Upon opening this door, the bed could slide out the window opening on a track and cantilever out the back. The bed would be enclosed in a weatherproof fabric, like the pop-ups. Support braces running down the rear bumper could be installed , if needed.
Once parked with the bed extended, additional space would be opened up in the bedroom area, perhaps adding a second bed under the first. While traveling, or in cold weather, the sliding bed could still be useable in the stored position. The nice thing is that it doesn't require structural modifications to the coach body/frame.
Works for me. Since you're only going through the window area, you should be fine.