I will second the statement about NO silicon seal ever on the outside of an Airstream. It sticks well enough that it is horrible to get off but not well enough to keep leaks sealed up.
You might want to think twice about treated plywood. I replaced all of the plywood on my floor with treated plywood and very shortly after I did I discovered that the newer varieties of treated plywood have chemicals in them that are extra corrosive to metals, especially to aluminum. I had to take it all out and start over. In the few weeks that it was in place I could already see corrosive action on screws and etc. I used a plastic type of plywood replacement. If you are just repairing parts of the floor I suggest that you just use a good quality regular plywood and make sure your leaks are fixed.
Only he who attempts the ridiculous can achieve the impossble.