I have cleared away all the rotted wood on the passenger side of the trailer. Now I'm going to explore what is under the bench below the slide-out: I am crossing my fingers that I can get away with cleaning the affected areas and begin my covering.
As far as completely rebuilding & re-engineering the floor: that is a little out of my comfort zone as far as materials are concerned, and I don't want to add too much weight. I know it is the ideal repair, but it would involve a complete gut-out of the trailer, and that is not in the cards.
Main question: If I add wood on top to strengthen the whole assembly, should I go with another 1/4" or should I add 1/2" plywood?
Also, what type of material should I use? Any water proofing/ mould inhibiting (etc...) stuff I can use? What is safe?
I do want to add some more supports around the perimeter in order to have something to carry the weight of the new layer, as well as give me something other than 1/4" plywood and foam to screw into. I was thinking of getting some square aluminum tubing, and cut it into pieces, and screw those pieces onto the existing aluminum frame at key locations.
this is the view from outside looking into the passenger side front access hatch:
