Oh my dear, I checked your profile and it says that you are in Great Brittain. If the window is not curved you might be able to buy double strength glass cut to size from a local Glazier or auto repair shop. Most modern caravans use "pebble glass" which does not break into jagged shards, but into pebble sized pieces that are much less dangerous. (My head went through a truck's side window and all I got was a bra full of pebbles. ) I do not know how, or even IF Pebble glass can be cut to size after manufacturing, so I would have tinting film added to both sides if I chose window glass.
Another option is to use polycarbonate material (common brand is LEXAN here in the States). It can be drilled and edge finished for operating windows, and in thinner sheets - 3 to 5 millimeters - will bend enough to conform to the curve on a side panel of an Airstream.
Another plastic is PLEXIGLASS (Lucite is a common brand). It is less expensive, easy to fabricate, and scratches easily. It also discolors, and will need to be replaced in 5 years or so if exposed to a lot of direct sun.
Your Airstream, your choice.
An Airstream dealer can get a window drop-shipped from the factory, but the cost, egad!
Today is a gift, that's why they call it the present.