This is the front/bottom of the cabover canteliever, right? Not the part next to the roof?
If so, assuming the construction is similar to my '98 Coachmen, you can get access to the other side comparatively easily by popping a few fabric buttons off the lower buckboard sort of panel between the bunk area and the front wall/ceiling of the cabover, and then removing the screws underneath the buttons. This buckboard then comes off. Underneath is probably some fiberglass and a sort of wooden cage framework, with the Filon and some thin plywood or something (which may have decomposed) backing it.
I've attempted to repair a couple of (smaller) cracks in mine by epoxying a couple layers of fiberglass cloth to the back of the Filon. In this case it looks like the crack extends all the way to the edge, so you'd probably need to remove the aluminum corner moulding channel and the edge seam underneath the bunk to get access to the back. I filled the front of the crack on mine with thickened epoxy (with a white collodion filler), which kind of sort of blends in if one doesn't scrutinize super closely. I still need to sand things a bit smoother. It does look like it will work for me, at least for awhile. Time will tell.
I also didn't like the cage-like framework Coachmen used, and constructed a replacement frame more akin to barrel staves—a bunch of thin slats butted together over a set of plywood formers, sort of like a wooden boat construction. I put a layer of fiberglass on the front and back of this to tie the slats together. It was a good bit of work to construct, but the result is sturdy but lightweight. (It's not the prettiest construction, but that's OK since it's hidden.)
If the crack has been around for awhile, there's a good chance of at least a little rot etc. to take care of. Hopefully the angle is such that not much water tends to get in and collect.