Likely plywood. I have a Chateau 24F with a step up to the bed area in the rear. Does it smell musty? Maybe a water leak from the shower? Also make sure your outside storage compartments are tightly closed and locked.
By "spongy" do you mean soft? If you press on it with your hand does it give? Note there is a thread/discussion on here regarding the vinyl floor covering being wavy and having bubbles. If that's what yours is like, it's by design. Thor does not glue the vinyl supposedly to allow for expansion/contraction to prevent cracking.
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