Use regular plate glass-
I'd use untempered regular plate glass, unless I was replacing ALL the glass. You're only talking about 1 pane, not the whole trailer, and its not that big of a deal to have one thats tempered.
The safest glass to install is tempered safety glass, with an applied layer of window tint film- which is available in totally clear as well as different shades of smoke, bronze, and metallized reflective tint. The tint will hold together the shattered glass if something happens where it breaks in transit. Otherwise, the shattered glass falls, allows no security, and requires immediate attention, which may not be available.
Tempered glass is harder to break, but also has some negative characteristics, such as temperature changes shattering it, and when you want to shatter tempered glass- you can hit it with a baseball bat and it won't shatter, but tap it in the corner with a spoon and hardly any pressure, and it shatters completely. Installation in the frame must also be done carefully- any contact with a metal edge will result in shattering when you start vibrating, bouncing your trailer along country roads.
The other easy and less expensive alternative, is laminated safety glass- this is what is used on your front windshield of your car- the sides have tempered. The glass is basically 2 layers of plate, with a layer of plastic laminated inside- when you look at it from the side, you can see its composition. Its made so that when it gets broken, it stays together, and glass shops usually stock it and can cut it and sell it immediately, where tempered has to be patterned, cut, sent out for heat tempering, and returned. I had a place that was huge and did it in house, but most shops send it to a big tempering facility, and they have a minimum charge that will far exceed the value of the single window you're replacing.
Be sure that you get the same "color" as a lot of shops stock different tints- even though they don't look like they are- its going to show even a mild tint when you install it- order clear. And I'm not so sure Airstream was installing perfectly polished edged glass on our trailers- especially when they have an assembly line and quotas to meet- if it gets covered, the edge is not a quality concern...
Good luck and let us know what you choose.