Way more than a simple repair to do correctly.
The luan, just to the right of the corner piece of plywood is delaminating and pulling apart. It looks to extend rearward, beyond the remaining fiberglass. You will never get a flat, strong bond as long as that delamination and cupping exists. It should be removed. Once removed, you will probably find the remaining layers are too thin to provide a good base. Maybe not.
To do this right and make it look decent, the trim pieces need to be removed, the remaining fiberglass needs to be removed back to a point where the luan is solid, then fiberglass matting can be applied, finished, painted, and the trim replaced.
I haven't even talked about the high likelihood of dry rot needing to be addressed in the remaining wood panels and studs. The bottom of that open area looks really suspect. It's been exposed to moisture for quite a while, IMO.
Lot of work.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.