I mounted a Penguin between the vents in my '59 Tradewind. You have less length, but only two vents to worry about instead of three.
Mounting the A/C between the vents will center it above the axle, which is a good thing. You want to pick a location between two ribs, which are easy to find by looking for vertical rivet lines. You might want to remove the interior wall sheeting on one side to make it easier to run the power and drain line. The drain line will probably end up in a wheel well.
The instructions that come with the unit will tell you to use wood to frame the space between the ribs. I used aluminum channel, which I think is less likely to rot. It's not a hard project.
And you will be pleased with the results. Your Airstream will look happier with the A/C unit centered over the axle.