Welcome to the group.
It should be stamped somewhere up around the tongue, usually on a vertical surface like the frame members going to the hitch. With the age of yours though, it could be obscured under years of rust and/or paint. You might try a wire brush on there and see if you can find it.
On newer models, it is also often on a sticky adhesive sticker on the very bottom front streetside of the trailer.
The VIN is also usually somewhere on the inside of the RV, too, but again, with a '69 it could have been lost years ago. Look in the cabinets for a sticky tag.
Worse case, if you can't find it, and you have a title showing a VIN of 1885 or B1885, buy some metal stamps and go for it!