The older Armstrong AC units are considered to be well worth the effort to repair by many. I'm not sure that is what you have, but here is a discussion by someone else who had one on a 1972: http://www.airforums.com/forums/f427...c-80847-2.html
They did decide to replace with a newer unit in the end. If you go that route, the things to look for are:
- A drain pan that fits Airstream and connects to the existing drain hose
- A nonducted type AC, since your trailer doesn't have the ducts built-in that some new Airstreams have
- What size? I would think that a 27 is getting on the margin for moving up from a 13,500 Btu to a 15,000 Btu if you go anywhere it gets hot. Especially in the sun.
Other than that, all current ACs are standard RV units.
My advice would be to get someone to look at the original Armstrong, if that is what you have. If that doesn't work out, then you can look at replacements.
Good luck, and keep us updated on your project.