If you cut through the insulation where the bulkhead goes and fit the 1/2" plywood to the wall and mark the wall with the outline of the bulkhead. Use the lines to drill holes for screws from the inside.
Run a bead of PU sealant / adhesive in the lines and press the bulkhead into place.
Run screws from the outside to hold the plywood in place while the glue cures.
You will not have to worry about the screws working loose as the sealant / adhesive will hold it in place and also stiffen the wall and become an integral part of the trailer.
I did this three years ago in my Scamp
and have over 20,000 miles on it with no cracks or problems.
Here is an example where I installed the overhead cabinets to the shell and the bulkhead that forms the front where the bath and storage is located. The bulkhead was installed using the method I outlined earlier. This bulkhead is 1/2" plywood.
The cabinets are an actual member in reinforcing the shell as they are also screwed and glued together.
The resulting strengthening of the walls with the bulkheads and cabinets make the shell stiff enough to not need vertical braces and allows for the twin beds in the rear.