I have no idea, but it is so close to the classic
Boler and
Trillium pattern that I suspect that it is from somewhere in North America. The coupler is also a style which would not have been used in Europe for many years (although of course it's an old trailer).
The owners, on the other hand, use terms such as "caravan", "perspex" (for acrylic, a.k.a. Plexiglas ®) and "cooker" (for stove), so they presumably have a European background. Maybe they did bring it from Europe?