The lower half is easily "wracked" (twisted) if the trailer is not level.
In our trailer, I level it by using a level on the "trim" (seam?) at the bottom of the lower half between the side and the sheet metal that is below the floor of the trailer.
If I get that level, front to back and on the rear of the trailer, from side to side, then I know the floor is level, so the rest has to match that.
Now, if the door does not open or does not stay shut with the trailer leveled, there is a simple adjustment you can make on the latch. It's done with a screwdriver and you move the latch in or out to make it catch properly.
My lower door latches correctly with the top up (and the trailer level). With the top down, the latch does not quite engage and the door does not want to stay shut. Clearly, the weight and stress of the top in the up position effects the opening there. Since it's not important that the lower door latches with the top down, I don't worry about it. The upper door holds everything shut in that configuration.
- Jack