Building a false wall inside the ramp door is a pretty common way to handle it. Here is a link to a picture (not my trailer or picture, sorry about the orientation)
http://www.rctech.net/forum/attachments ... 10-002.jpgThere are many variations on this, but usually it is building a false wall inside where the rear door closes, and then building in a door (opening, sliding, etc) into it. It is also a way to build in a wall mount air conditioner without cutting into the walls of the trailer. It gets built into the rear false wall next to the door. My friends trailer is that way, but unfortunately no pictures of it.