From looking at my trailer, the bottom half of the trailer is molded very similar to the top, so the bottom is fiberglass with a gel coat finish on the outside, underneath. Bigfoot
floor problems are not too common. The fiberglass has plywood on top. Tom's 1980 has Styrofoam under the plywood and it sounds like it is between the fiberglass and the plywood.. I don't know if mine has foam or not.
In any event, the first thing I would do would be tear out the vinyl and get down to the plywood. It sounds like you ill have to remove the dinette seat bases to get to the plywood. Then remove any rotted plywood. If you have foam between the fiberglass and plywood it should be pretty easy to separate the rotted wood.
This is a new area for most of us Bigfoot
owners so let us know what you find.
Make sure any leaks
have been repaired. A leaky water tank, pipe fitting or window is what started the problem.