On the driver's side front corner there should be a placard with that information stamped into it.
7:00 x 15" was the original size, but it is difficult or impossible to find tires sized that way. These tires were narrower than today's radials, so be careful in your selection, so that the tires will not rub and so that they easily fit into the wheel well.
This is a link to an Owner's Manual for your trailer. Tires are on page 22>
https://cdn.airstream.com/wp-content...-manual-51.pdf
An article you might find helpful>
https://www.etrailer.com/question-29678.html