Not to completely hijack the thread, To GPW
What is a proper "dip edge" ? Where would it be relative to the shown trim? Could you provide a drawing?
It seems that no matter what you do, an outside L shaped trim along the bottom edge will always have the possibility of catching and holding water. And trim along the top edge does not guarantee a seal.
If you are using the trim as accent eg complimenting the trim along the top edge or actually securing the side wall to the trailer, would a flat piece ending at the end of the side wall be better as there is no where to collect water?
For a typical 3 layer wall, would it have been better to have

Click for a bigger view
- no overhang and no trim, just a well coated/sealed outer surface and edge of outer wall and middle wall. The inner surface of the outer wall is glued to middle wall [probably the most common solution (??)]
- a well coated/sealed overhang and no trim? ( Red area of outer wall in picture ) This might just be moving the issue to the middle wall's edge
- would flashing ( blue vertical or dark blue/purple L shaped ) inside the outer wall be good? Or is such a barrier just another problem waiting to happen? Or just too much over building?
No matter what solution is provided, the weak link is the plywood edge, be it the top or bottom.