We spent a lot of time trying to figure out a way to lock our rear door since we wanted to be able to lock it from the outside as well. We came up with this solution that has worked well for us so far. I think we may upgrade the lock cylinder if the weather starts to affect it. We posted a video to our channel showing it off:
First, thanks for posting your rear door lock solution.
I just completed the rear door lock on my bus using your method, with a couple small modifications since there are some small differences between your back door and mine.