Your latch isn't coming off (despite undoing the four screws holding it in place) because (presumably) you still have the outside handle attached and that keeps the whole thing locked in place. As o1marc mentions, it's not easy to remove the outer handle even after you get the set screw that holds it in place out. Early on in my build, I just cut the outside handle off entirely with an angle grinder and cutting disk and riveted a piece of sheet metal over the hole. I don't use the back door as an entrance, so I don't need to ever be able to open it from the outside. I kept the inside
handle and the latch functional - this arrangement makes the door as secure as possible (no lock to pick) but it still works as an emergency exit door, since I can open it easily without having to fumble with any deadbolt arrangement.
My inner latch was also in the way of my bed, so after I insulated my door with 2" of XPS, I shifted the latch to the left and down a bit, as shown here: https://www.skoolie.net/forums/f11/r...tml#post429664
. Since the latch was now not next to the edge of the door opening, I actually have it catching on a little tab welded to the corner of my bedframe.
You could do something similar on your bus, although a problem you'll face is that if you go up more than a inch or two, your latch will be over the window and you won't be able to screw it into the glass.