The subfloor in Airstreams is plywood (In the early 2000's they unfortunately experimented with OSB for several years on 22' models, and maybe some smaller ones, stay away from these.) and will rot if continually exposed to leakage.
The leaks may be interior plumbing or from outside. It is so common in used RVs you must check the subfloor for moisture and the only way to do it is lift the floor covering (owner won't like that and I wouldn't either) or probe through the edge of the floor covering near plumbing and perimeter of trailer with a moisture detection meter, like home inspectors use. You can get one from Amazon.com for about $30, and it will be very useful to maintain your trailer in the future.
The floor could still be rotted but dried out. That would be a visual inspection, look for clues such as uneven floors.
Look for cracked, unmaintained sealants, a direct cause of leaks. Panoramic windows leak more often than not, and its usually the tiny ribbon of sealant at the bottom curved section (this is an ongoing maintenance item, put a fresh line of sealant here, maybe yearly).
Expect corrosion of frame, shell, and exterior fittings, especially if trailer has been near the coast. Decide your level of acceptance, mines pretty low.
I can tell you from experience my 2012 Airstream is a more nicely constructed trailer with better fittings and more seals than my 2007 Airstream was, both were very good. Ongoing sealant and corrosion maintenance is part of ownership and easy to do if kept up.