Having had 3 Bigfoot's I believe there is just a chunk of plywood bonded in underneath the skin with lag screws. I don't know about the Outfitter system, but it is probably pretty similar. Not the best way, but it seems to work, how many fiberglass boats hold up big V-8 engines in a similar fashion.
One time I had a mule deer turn back into me and she took both jacks right off. There was almost no significant damage to the fiberglass body where the jacks bolt on, just slightly enlarged holes and a little splintered fiberglass. I got new jacks thru insurance and just drilled out the holes from I think 5/16" to 3/8" bolts, made sure I had clearance thru the fiberglass and bolted them back on. A rather simple repair, on aluminum frame rig I don't want to think what would have happened. My advice, make sure you have spring loaded tie downs on all corners, I don't like the idea of a fixed tiedown with that type of construction. Just my opinion