New Floor And Frame Detail
The old plywood floor was 1/2", I would have preferred to use a thicker plywood, but since everything had to fit together exactly as before, using a different thickness would have meant the skin would be too short.
After the initial fitting, the underside of the floor was treated to 3 coats of a good oil based paint, the first coat thinned 50% for good penetration.
After all the construction was complete, the underside received 4 coats of rubber based undercoating (wood included) as I chose not to install an underbelly.
Based on the same thought, I used an asphalt/rubber based roofing sealer (the material the young folks use to keep their cars from rattling when they crank up the stereo!) The material is actually a roofing product, in my case, used to isolate the wood floor from the steel frame, to seal the fasteners, and act as a drip edge for the inevitable spray produced from driving in the rain (it never rains when you're on vacation...Right?)
The thickness of the material was accounted for by the frame being just a slight bit shorter than the original.