Isn't the fact that the floor is rotten enough to show you that serious moisture has been into the "basement"? Remember, all Airstreams leak, it's only a matter of how much and for how long.
Unless you've got x-ray vision, you won't really know about rust on the frame until you look, so if you intend to buy it, be safe and budget on having to do a frame off restoration and a new floor.
Good luck with it!
“Courage is being scared to death, but saddling up anyway.”