Okay this is the response with links. It is very positive and the suggestion is the EU won't tolerate independent attitudes of local Prefectures:
Please find below the reply to your enquiry. Please note that the advice given by Your Europe Advice is an independent advice and cannot be considered to be the opinion of the European Commission, of any other EU institution or its staff nor will this advice be binding upon the European Commission, any other EU or national institution.
First of all, we would like to point to your attention the following page of the Your Europe portal where the procedure is extensively described from an EU law perspective:
(Registration > Formalities and documents)
The same rules apply to your caravan. Note that this information is intended for the private owner moving his private vehicle to another EU country.
In the light of that, obviously, in reply to your question, the French prefectures are not allowed to do what they want. As a matter of fact, the competent body is more the DRIRE than the prefecture, since it will be the DRIRE that will deal with the type-approval of the vehicle. For instance, as is contemplated by EU law, it will check that there is an EC certificate of conformity or otherwise verify concretely that the vehicle meets the conditions for type approval (“reception à titre isolé”).
See at this page (in French) the French procedure, as described on two different websites (one official, the other not):
(see the two relevant sub-headings, depending on your situation: “Immatriculation d'un véhicule neuf acquis à l'étranger” and “Immatriculation d'un véhicule d'occasion acquis à l'étranger”
If you experience difficulties with the registration in France of your caravan, and find that the French administrative practice does not conform to EU rules, please contact us again.
I have a reference number for my inquiry I can share if anyone requires this.