I am no Boler
expert, but I have carefully considered how to replace the plywood that is sandwiched between the floor and the exterior bottom of a Trillium
If I had to do it, I would pull the trailer off the frame, and cut the bottom of the trailer. It is possible that you could get away with just cutting down one side, and then across the trailer from side to side, but it may be necessary to cut out an entire patch.
The advantage of cutting from the bottom is that most of the seams will be out of sight.