I have not heard of 3/4" ply in 60's era trailers. The number I usually hear is 5/8". It seems that the only way to know for sure is to measure your own every carefully, as the "standards" used at Airstream seem to have been pretty fast and loose, and varied between California and Ohio.
It seemed that I had 3/4" in my 70's era trailer, bought and installed the wood, and then found that it was slightly too thick for the C-channels to wrap around. Stranger still, I bought new C-channels, and these had an even smaller dimension than the originals. When I got it all installed and bolted down, the holes in the front plate on the frame matched up with the holes in the shell, which seems to verify that the original ply must have been pretty darned close to 3/4.