I believe airstream will honor the warranty, which means that since this comes after the warranty, they will not cover the repair under that warranty. I could be wrong, however.
Depending on how long the leak has been going on, and more specifically, how long the plywood has been wet, You may not have to replace that section of the subfloor unless the plywood has gone soft or started to rot away.
There is no cheap or easy way to replace the subfloor short of first removing everything inside the trailer that is above that section of the floor. Because this type of repair is so labor-intensive and expensive, I am not surprised that your insurance company does not want to pay for it, nor will airstream if they don’t have to. From what I gather on the forum, the way airstream regards the Warranty is that you have been “given” two (now three) full years to find any of the many factory defects these new trailers are known to be riddled with, after that time, you are on your own.
The factory will, on your dime, be happy to replace the floor for you if you like. It will be a little comfort for you to know that starting with the late 2020 models, airstream finally switched to a waterproof composite floor in all of their trailers.
If they don’t want to pay for the repair themselves, perhaps you can plead with them to cut you a deal on the repair, if it is needed.
Another option to crunch the numbers with is to consider selling your trailer now as is, and buying a new one with a new composite subfloor, and see what that would be compared to paying for the repairs to this trailer now
It would be great if you could buy insurance that would cover manufactures defect‘s after it runs out of warranty. In the case of airstream trailers, the insurance would be so expensive, no one would buy it. That or the insurance company would soon go bankrupt. It may be telling that since many manufacturers will try to upsell a new customer on an extended warranty policy, thereby increasing their profits, Airstream does not offer such an extended warranty.