Not having a ramp makes it easier.....my first thought was a ramp, since I assumed you wanted the back to open to bring in something BIG.
Personally, if I was to go thru the trouble of cutting into the back I would definitely have it so I could just open it up from the top, preferably with hydraulic "thingies" (sorry, I'm just a woman who doesn't know all them fancy names for things), so it doesn't just open and knock some sense into me and have me a nice back porch (with a canopy for rainy days).
Also, having a nice level spot where you can work on whatever it is you plan to bring on board, without having to kneel down and/or bend over. I picture having the "new door" sheet metaled for ease in cleaning and walking on and of course with rubber sealing all around for creating an air type opening and protection for myself while it's open.
Darn, sure wish we had the health and time to do something like that on our bus.
Sorry I don't have pictures to show you what I mean, but it seems pretty straightforward.