Generally, use of a power washer on a "old dirty Airstream" is frowned upon. The seams are caulked to prevent leaks and the caulk tends to dry out over time. The pressure of a power washer forces water between the seams & compromises the caulking even more which in turn causes leaks - the nemisis of all Airstream trailers!
IF you are recaulking all
the seams or doing a major restoration & taking the interior panels off - where you can inspect for leaks, then it would probably
be okay. But I wouldn't do it as a means of just washing it.
We had the entire interior out of our '56 while restoring it and power washed the interior panels while stripping the many layers of paint. While the interior panels were out, we were able to check for leaks in the exterior skins at the same time. We also re-caulked the entire exterior during the process...