Carl beat me to it. Heavy duty 7-1/4" saw fitted with a nail blade
(preferred) or a cheap carpentry carbide blade. Depth set to 1/16" less
than the total thickness. Mark out the open spaces, cut out all the
material. You want to leave a 2" wide strip of doubled plywood with the
screws in it. Use a super magnet to find the screws, cut them off with
sawsall or run the saw crosswise and expose the head.

The brute force method is to flip it over and beat it off with a sledge
hammer.

Karl Townsend wrote:
"The Kid", my 25 year old, got a fifth wheel travel trailer for free. So
far, he's taken the backhoe and ripped the whole top off. Now there's a
ripped up floor on what looks to be a decent fifth wheel frame.

It looks like they used self tapping screws to attach one layer of plywood
to the frame. Then a second layer bonded over the top with zillions of
nails. The deck is not prying off the steel frame no matter what he's tried.
He's about to say UNCLE and send it to the dump.

Any suggestions on what to try?

Karl