Joe, Sorry I checked this thread out late, but I want to say good job on the floor installation. Using extra screws in the installation is a big plus. The plywood definitely does contribute to strength and stiffness to the floor platform and the whole structure. Its the same principle as attaching sheetrock to flimsy 20 gage metal studs to make a sturdy wall in the construction of buildings. Using the West Systems epoxy will make the plywood somewhat stronger, but it won't make it water resistant unless you applied at least 3 coats. And, it won't make it rot resistant unless you completely encapsulate the wood. In the 80's, I used West Systems to build a racing sailboat so I am very familiar and experienced with it. Fact is, I used some tonight to fix a ferrule on a windsurfing mast for my nephew.
In my trailer, I wanted to prevent wood rot so I considered two wood preservatives I've used in the past. One is the green copper napthanate basically the same stuff in wolmanized lumber. This should never be used on an Airstream as the copper in the preservative will cause big problems with the aluminum skin. The second wood preservative I've used in the past was Olympic Stain wood preservative. Olympic Stain is primarily mineral spirits, boiled linseed oil, fungicides, insecticides, and water repellents. It seemed like the best choice for my application since I planned to put down foamed backed sheet vinyl flooring. Before replacing the finish flooring, I flooded the plywood with the Olympic Stain wood preservative, then let it dry for a couple weeks before installing the new vinyl. It was a bit "fumy" for the first month or so, but after 2 months no noticeable paint smell.
This was just a thought I had while reading your posts, and I wanted to pass it along plus, I wanted to say good job. My hat is off to you! To bad I don't still live in Austin, I'd love to drop over and see your trailer.