Dave wrote;
The aluminum side is dry and the interior is damper and swelling.
I agree with Dave, most warping problems with wood can be/are traced back to the finish, or lack of.
When dealing with a piece of wood that has a tendency to warp, most would recut the piece, and glue it up so the grain is alternating.
The more common solution is to use plywood, as the layers are alternating, making for a much more stable piece.
When building something from plywood, most often it is glued, screwed, or nailed to a rigid frame, thus leading most to believe that plywood is a super rigid, unbending product...but that is not so.
Lay a sheet of plwood on the ground, come back in a day or two, and it will be anything but flat.
The reason is the much higher moisture content on the side of the ply facing the earth.
Now, with the ply installed as a door on a trailer, with no rigid framing, there isn't much to hold it flat, just the hinge and the latch.
Since the latch is usually centered in the middle, the area just above, or below it is where the wood will be most prone to move.
The solution, as Dave pointed out, sealing the wood.
Sealing is more than just slapping a coat or two of varnish on the part you see.
To be effective, all 6 sides must be sealed moisture tight (when the wood has an even moisture content...not warped).
When I seal a piece, the coats required will vary from 3-6 (more if I want a super deep finish).
The coats must be even though, not one on the back, and 4 on the front.
Try to imagine that the wood must resist a puddle of water laying on it overnight, no moisture must get through the finish.
If you make it so, it will not warp, or rot.
Be sure to check the hinge placement, just a little out of plumb will cause the door to be out of plumb, creating a gap.
Also check the window installation.
If the door was slightly twisted when you screwed it down, the window will help it hold that shape.
In a safe, controlled experiment, I would first use a small electric space heater to warm/dry the air inside the (closed up) tear.
A de humidifier could also be used.
If an excess of moisture on the inside of the tear/door is causing the trouble, when it dries, it will likely "spring" back...unless the window installation is holding it back...
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...