On my #2 build I needed a hard points to mount the surface mount door than I purchased for the trailer. The window was a clamp mount which holds the window in place simply by having a outside flange around the window perimeter and a inside ring. By screwing the ring to the backside of the window it just clamps the window in place to the wall, no wood needed. I found only one door that was a clamp mount style. Unfortunately, it was a small sized door and cost the same as the larger surface mount door. So I chose the surface mount door.
Then I had to come up with a way to mount it to the wall. Not wanting to install wood per usual knowing the expansion issue I decided to try a new method. Sorry I have no pictures so I will have to describe what I did.
I used my soldering gun with the normal soldering tip and a piece of scrap 1 3/4" flashing I had laying around. I used the flashing as a straight edge and I used the soldering gun to cut a 2" deep 1/4" wide groove into the side wall. I stopped the cuts just past where the radius started for the corners. Then I cut some 3/16" hardboard into 1 3/4" wide strips. I cut the strips to the length of the grooves and then slid the hardboard strips into the grooves. I cut the strips 1 3/4" so that I could fill the grooves back up with caulk and seal the hardboard into the wall. Now I was able to screw the door and the inside trim ring to this embedded hardboard and there was no wood to telegraph through the canvas. I have to say it worked out better than expected. I did have to buy longer screws though. You can see me using the soldering gun at this point in this video
https://youtu.be/IOTRwcs6oT4?t=1m55s .
Good luck, George.