Damp is probably the single biggest issue with secondhand caravans of any make. It's usually not " does it have a damp problem" but when. Did or will it have a damp problem. Unfortunately you can't assume there is any brand or model that can be guaranteed to be free from damp issues
The fact is manufacturers can build perfect caravans, and there are enough examples where some customers are completely happy, so this proves the manufacturers can do it, but customer satisfaction surveys also prove it's a lottery, and like the lottery there are plenty of losers that prove the manufacturers don't take customer satisfaction seriously enough. Their casual attitude destroys dreams and resale values and costs customers time, loss of access and money.
Damp does not have to be death of a caravan, if it's been detected early enough and been correctly repaired then the caravan can have a good service life, but good repairs cost money, and depending on the age of the caravan, some parts may not be available.
Unless you are a gluten for DIY projects (and some people are) you need to buy wisely. Get yourself as much protection and leverage that you can.
I always advise that if you find a caravan you like, then it pays to have an independent pre purchase inspection. Members of the AWS can do this, but have it done before you agree to purchase. If you don't like the report walk away. If the owner baulks at the idea of an independent inspection ask yourself what are they trying to hide? and walk away.
As others have suggested, use some finance product to make the purchase, as then have some other organisations under section 75 Of the consumer credit act, to leverage for support if you do have any after sales problem.
Provided you do buy from a trader (Retailer) you have some powerful rights under the Consumer Rights Act 2015. It's worth reading up about the CRA.
Some different makes and models do seem to have common issues so it can certainly be a valuable exercise to gen up on those frequent issues or know how to spot the problems.