The sign is less than 12 months old. However, it not only prohibits caravans, it also prohibits commercial vehicles, lorries and trailers. So self-built 'vans with windows' are prohibited. I suspect that if you go into the bylaw or TRO that created the prohibition, somewhere in the chain will be something that defines a 'caravan' to be a 'vehicle, whether or not motorised, constructed or adapted for habitation'. That said, there were four obvious PVCs parked there when I last visited!
The most annoying thing is that this car park is huge and even a dozen motorhomes overnighting are not going to cause any issues. Like
@Debroos, Jan and I are local and, before the lurgy struck, we used drive out there in a panel van adapted as an office and set up for a few hours of stargazing/astrophotography. Even in the depths of winter, we regularly shared the car park with motorhomers, who either kept to themselves or showed friendly interest in what we were doing and so were never an issue.
Now, AFAICT, this car park is part of the Mount Edgecumbe estate, which I understand is jointly run by Cornwall County and Plymouth City councils. However, I thought that neither council had direct control and that everything was done via a joint committee -- which brings CCC's prohibition into question. That said, I thought it only fair to warn people of the changes.