If you have a legitimate
title you could find the production serial # on the RH top side of the tongue. You must search for it with a series of welder's stamps and a 2 pound hammer.
Held to a straight line.lightly sand and repaint after search.
Actual VIN#s for RV trailers were introduced in the early eighties. Before that, some trailers only had a paper glued to the inside of a cabinet door with manufacturer's info and serial#.
I'm not sure what your particular trailer had, but I think my 1300 could have been one with it on the cabinet door. The door or the paper on it could have been lost over the years on yours.