The fibreglass should be part of the outer wall skin not a removable sheet, unless things have changed......... the addition of the T section would cause a change in wall width for rest of wall panel and possibly cracks further along ...
Thinking more deeply on the issue, since the whole panel is structural the recessed section of the inner ply wall and the adhesion of the styro to inner and outer panels is most important to the integrity of that whole wall, more flexing from that section of wall could cause issues elsewhere if not failure on the repaired section. ......................... thats just a observation, more I think of about it, the more I would be inclined to actually just remove the whole wall repair with new styro and ply and refit, trying to place the inner ply correctly, slotted in place at the top, side and bottom sections might not be achievable, hence take the wall off, a big job for sure but at least should keep the integrity of the whole wall and possibly be easier than an in situ repair................ The use of marine ply could cause you issues when the van moves as it won't flex in the same way as the rest of the wall, you need to keep the materials of similar construction so they all flex in harmony............ the join of the inner wall section would need to be staggered in that the join of the styro and inner ply wouldn't be in the same spot a sit would certainly open up.
I have done some small repairs over the years mainly to replacing rotten timbers or internal skin and as such and they always end up bigger finnicky jobs than first envisaged and I often wish I never started....
This is assuming the wall in question is one complete panel from front to back and not a join in the middle ...................
PS: Found it;;;;;;;;;;;;;;; ... Link to the how and why of the wall build .............
