Templates..

Make sure you measure and take tons of pictures as you do the tearout. Also, you will want to keep some parts of the trailer so you can make templates out of it later. The angles may change some but over the course of the build you will forget how some parts fit together and this helps.

Here is the rear upright. Angles had changed so I used Cardboard strips  to make a tight fit.

Here is the wall before I cut the panel. I made a template from 3/16" x 1.5" strips  Then drew it out on the paneling.