I removed the top bunk from our 26BH, piece by piece, so I have a good idea of how they're constructed. They use a reasonable amount of 2x2's, so the frame itself is fairly robust, at least by trailer standards. But, as you discovered, the skin is thin.
Half inch ply would get the job done with extra peace of mind. I tend to over-engineer things, but I'd be happy with a sheet of quarter inch luann ply up there on top of the existing skin.
Six hundred pounds, though. I suppose they tested and ran the results by their legal team, but there's no way I would sleep in the bottom bunk with two 300# people up there
Or, three 200# people
.