Ask the seller if they have updated Cris when they bought the caravan. It's optional & plenty don't. If it has not been updated then you need to pay for a check yourself to ensure it has not been stolen at sometime during it's lifetime.
It does depend how much you are paying for the caravan. Several £1000s then do a check but once caravans get old enough to be worth a few £100s then few will bother with checks or updating.
There is nothing to prevent seller emailing Cris number to allow potential buyer to pay for his own check. I would guess it's simply seller cannot be bothered. Ie if you want caravan come here look at it, pay for it & take it away otherwise don't waste my time. Ime plenty of sellers take this attitude. You ask them simple questions on ebay & they do not even seem capable of a coherent answer.
The op in fact asked the seller the wrong question & assumed seller had updated Cris when he bought it which may not be the case.