Its to keep the raggers out.
No one wants to pay to stay on a domestic travellers site , for the weekend , and the toxic ambience it will exude.
Normally, or they used to when I had a touring caravan, state NO adria, hobby , branded caravans etc.
And no sign written vans or flat bed vans , tippers etc.
If your van isn't registered as a motor caravan, as long as it is "perceived to be respectable " with windows , shouldn't be a problem.
I've never had to produce my log book , proving "motorcaravan".
Not even for ferry bookings.
Maybe their automated system cross references with the dvla??