Replacing bottom board

[Bigmike]

After 20 years the underside of my 28x80 mobilehome is in shambles. I have no one to help and I'm sure this is a repair that would cost thousands. So why can't I replace the plastic sheeting with plywood? Laying the insulation on top of the plywood. And securing the plywood to the metal frame with screws or to the floor joists. What would be the results?