If you want to know about the door construction, just take the window out of the door. it comes out with only a few screws. From what I remember from put our stained-glass window in is the the door is just as you thought. Fiberglass wall board, Styrofoam, Al metal interior panel. I don't think I would trust the door to be strong enough to hold a dead bolt in place if some one was trying to get in.
I don't think that all the locks you could mount will ever keep someone out that really wants in. there is always a weak spot, be it a window or a door, if they really want in your trailer they will get in.
On I side note: With my Jeep, during the spring and summer I always had a either no top or just the vinyl top on it. during this time I NEVER locked a door. I have seen many jeeps with knife slits in the windows because some one wanted in, so they just cut a window. I had mound several steel lock boxes to the floor and roll-cage and kept them locked with with all the good stuff in them. I know of at least two times that the jeep was "entered" and nothing was removed or damaged, just the doors opened and not shut. I preferred them to just come in and take a look around, and not make it hard for them to get in.