After many missteps, trying to replicate a thicker joist type Shasta floor, and even buying some bargain basement Chinese plywood that could literally be ripped apart with your bare hands.
Then we secured the plywood with 1-1/4" deck screws and countersunk carriage bolts to the metal frame. Each panel was coated with asphalt emulsion for waterproofing.
Finally, we reinstalled the wheel wells using 1/8" putty tape and hex head sheet metal screws from underneath.
Finally, after filling all the holes with wood putty, the chief helper manned the paint roller to add a protective coat of primer until we can install the final flooring material