There is a facebook group for these campers. Before you jump in and click on the link read my statement below.
https://www.facebook.com/groups/107346979296520/
Word of warning, you should use street smarts if you are serious about owning that trailer you spotted. You will be meeting professional restorers and re-sellers in your hunt for information. When you go to the facebook group not disclose the location of where you saw the trailer. Keep that information totally secret as you don't want someone coming in to scoop it out from under your nose before you decide what you want to do about it. Don't even talk about your find. Instead just ask generalized questions of interest in the vintage trailorboats such as is it hard to find the boats that fit them?, is it hard to find the trailers?, etc. Keep a poker face and do not show what is in your hand.
These are very rare trailers and not everyone you meet on the internet will put your interest above their own compulsion to have rare things and/or possibly wanting to make a profit from them. Professional people who are on the hunt for rare cars, art work, antiques, books, etc all practice being discrete and do not reveal their leads to a find and neither should you if you are serious about acquiring it.