John
Here in the U/K trailers,'vans etc have to be numbered as per the towing vehicle. Number plates can be bought at car accessory shops and it is only fairly recently that you had to provide some proof of ownership of the towing vehicle. Years ago, before this requirement, I had a plate made up to leave at the storage pitch when we were away in our 'van taking proof of ownership with me - the guy making the plate for me seemed quite bewildered by that. It had not occurred to him that someone might have false plates made to put on a stolen vehicle.