Most of the time locksmiths will destroy an RV lock, because they are not familiar with them.
A new lock, considering you paid little for the trailer, does not cost that much. That original lock is still available.
The old lock can be carefully cut off, so that you can open the door.
After you get into the trailer, check all the exterior gaskets. If they are original, they will all be shot.
74 and older Airstream trailers have axle failure because of the wrong rubber formula used in the rubber rids.
The axle, more than likely, is shot.
You can easily check the axle out within 30 seconds, without taking anything apart.
The following article will teach you how.
Dura Torque Axle