I installed a custom-fabricated support for the cab-over. Two steel rods were welded onto small brackets in the doorjam, which was then also welded to 3" steel plate. The steel plate ran the width of the fiberglass, and was screwed into the existing metal support. The wood and foam from the cab-over interior was destroyed by water damage, so it was all ripped out and replaced with new high-grade lumber, foam, and plywood. All in all, I'm very happy with the results and shouldn't ever have water problems in this area again. It's a shame that they didn't think to add more support here in the first place! I'm still not done, I could use some help in deciding what to do about the outermost layer, which is still heavily glued on. It's incredibly hard to strip off, but I was thinking about buying an electric planer to get an even surface to glue a new piece of plywood to. Thanks for looking!
Here's an album of the fix:
http://imgur.com/gallery/JP7aO/new