[HoosierDaddy]

Hi Sean...welcome.
I had a similar condition with my 1978 Newell. The problem is moisture that gets trapped below the wood. Check your bay door seals and your holding tanks for leaks. If you have a steel waste tank (I think that was common in early Newells) it may be rusted out on the bottom. My fresh water tank was Stainless steel.I removed all of the wood and metal in the storage bay AND the water compartment bay (after removing the water and holding tanks). The frame is square steel tubing that runs longways under the floor. I used an air powered chisel to remove the sheet metal leaving the square tube frame work (It is much thicker and probably still sound. If there are weak sections you would need to repair/replace as needed but that would be more involved since those tubes are a load-bearing part of the frame) A steel shop cut new sheet metal panels that I laid in place after I painted the frame with POR 15 paint http://www.por15.com/ I also painted the new sheetmetal with POR 15. I built a new waste tank from steel and coated it with POR 15 inside and out. After stitch welding the new steel floor in I caulked to make sure no water could enter from below. New RV's and semi trailers use a plywood that is laminated with fiberglass on both sides. That is what I used for a new floor. You could use marine grade plywood as well. After the plywood is installed Caulk the perimeter of that to keep moisture out from above. I finished with a layer of undercoating on the bottom side. I removed the aluminum sills along the bottom of the door opening and had the metal shop make new ones as well. Ten years later it still looks great!