I was out riding my ATV this weekend and we came to a tailhead & there was a bus sitting there. I went over and checked it out, they had the left side of the rear made into a 3rd door & could load quads in the back. Similar to the one inthis post
I had to have a look, it looks fairly easy to do, I think I may do this to my bus. I would have taken some pictures but I was riding my quad and didn't have my camera on me.