Hello Tkhogg and welcome to the forums. Congratulations on your new vintage trailer.
Floor rot is a common issue with Airstreams. The rotten floor you have uncovered is probably just the beginning of what you will uncover. Any exterior opening is an avenue for water to get in and drip down to the floor eventually. Just because you don't see the water leaking into the interior of the skin doesn't mean there are no leaks. Chances are the front window is or has leaked for a long time. It is probably leaking down between the inner and outer skins as you suggest. The floor is a critical component of the semi-monococque construction of the Airstream. A sound, sturdy sub-floor is critical. A rotten floor under the front window is very bad news for the overall structural integrity of your trailer. Rotten floor must be replaced properly by gaining access to the bottom of the floor by removing the belly pan.
To find the leaks, you can drill out the rivets that fasten the lower interior skin to the ribs. Use a drill with an 1/8th inch bit. You'll have to remove all the old insulation to be able to see where the water is coming from. Have a helper spray the exterior slowly from bottom to top with a garden hose as you look for the source of leaks. You can also go in the trailer when it is raining and look for leaks, of course.
If you have some time, have a look at the work I did on my '72
. Good luck and post some pictures!