Using the wayback machine website (which I love for finding old stuff on the internet), I did find the specs from the 1998 models which does list a 13 inch tire for you trailer.
http://web.archive.org/web/199807020...com/specs.html
For those interested, you can use the following website which has been discussed here before by AstroBruce :
http://web.archive.org/
then enter the trailmanor website (or others) to lookup, and find all kind of information on years past models, with surprisingly still active links to click on.
A ST175/80R13 trailer tire should not be hard to get. We stock them and sell them all day long for $60. Have you tried the tire rack or discount tire?