VIN numbering convention, at least for motor vehicles in the US, wasn't standardized until 1981. Your 1972 pre-dates any real standardization so my guesses are going to be exactly that.
"Typically", for that vintage, a trailer's unique ID number would be found on a paper sticker adhered to an inside cabinet door, or other location near the entry. There might also have been a printed (maybe not stamped or embossed) tag with a unique identifier stuck on the forward part of the camper's left side frame.
Either of those are not all that durable and may have decayed to an unreadable state by now.