Don't forget for the sake of a few quid you can buy a damp test meter which will give you a good idea rather than paying for a test to be done by a caravan shop
I have to disagree with the above posting in as much as a cheap damp meter is just that, cheap, and how accurate it is may be questionable.
Also if as a newcomer to caravans you dont know what readings you are supposed to get or where to test and how to , then it is worthless numbers.
Buying from a good dealer has a lot going for it as it will be covered by the CRA should anything problematic happen , and may well come with a guarantee as well.
If buying from a private seller it is essential that you get a mobile caravan engineer to give it a Pre Purchase Inspection, this will do several things:
1 it will prove that the van actually exists. If the seller refuses to let an inspection done just walk away and
forget it.
2 It will either prove or disprove that everything is actually working, which unfortunately means that some sellers are not as truthful as they should be, and actually not everything does work. I know just how often this scenario happens as a caravan engineer, and it is regularly.
3. It will pick up whether there is any damp which is a walk away situation, that's the last thing you want to be dealing with.
Paying for a PPI may cost a bit but may save you thousands in a bad van, and the frustration of dealing with it.