Unless provisions have been made in the regulations to cover specifics, both cars and trailers (caravans) must comply with the same rules and laws. As far as technical provisions are concerned, they are both treated the same, as road vehicles.
If, as you say, vehicles must be technically approved for use at the maximum allowable speeds anywhere within Europe, then cars and vans capable of very high speeds would have to be approved to their technical limit (to cover the unlimited conditions in Germany), and all other vehicles (trucks, trailers, caravans, etc.) to cover the respective highest speed limit applicable in any of the European countries (for caravans that would be 130km/h for France).
I think you will find that where top speeds over 250km/h are quoted, then these are the maximum speeds that would be achievable without the electronic governors that the manufacturers voluntarily fit.