Retail purchases of all kinds, brand new and second hand are covered by the Consumer Rights Act, which requires sellers to ensure the goods are described accurately, that they are fit for the intended purpose, and are of sufficient durability and quality to last a reasonable time without any new defects arising.
Especially when buying second hand it is vitally important to be aware that the adage Buyer Beware applies.
What this means is, it is the buyers responsibility to check that the goods are fit for purpose before they agree to purchase. You do that by asking questions and having the seller answer them to your entire satisfaction. I suggest you go further and have your questions written down and get the answers from the seller in writing or at least witnessed or recorded so you have corroborative evidence of the sellers responses and promises.
Framing your questions is also important. For example, if you were to ask "Is the caravan damp" on the day you ask, it might be perfectly dry, so the dealer could legitimately say yes its dry. What you should be asking is What is the caravans damp history?" the seller is legally required to answer as accurately as they can, and if they don't know they should say so, and you can either choose to leave the deal (which is what most sane people should do) or to accept the risk and use it to negotiate a lower price.
Dealers and traders are expected in law to answer such questions truthfully as they are considered to be in a position of trust because of their technical advantage and specialist knowledge. If what they say turns out to be incorrect they are in breach of contract because they have not accurately described the goods. If you have evidence of untruths you are in a better position to get some form of remedy through the CRA action.
The other important action you can take, is once having identified a caravan you would consider purchasing, is to have your own independent inspection report done on the caravan.
If a seller is reluctant to allow you to have the caravan inspected in this way, ask yourself what are they trying to hide, and very seriously consider if you want to take the risk of buying it. - A few pounds now might save thousands later.
Use some form of credit (even a credit card but NOT debit card) to purchase the caravan, then if there is a problem then you may have some come back against the finance company under section 75 of the consumer credit act.
You must have seen how many forum members have at some time had problems with a caravan from new, So it's highly likely that second hand ones will also develop problems, sadly it seems to be an almost inevitable consequence of caravan ownership, so you do need to take whatever action you can to protect yourself. We are lucky in the UK to have our Consumer Rights Act which comparing it to similar acts around the globe are surprising powerful, so read up about how to use it to protect you from bad purchases. Don't forget it's not just caravans, it applies to ALL retail purchases. They are your rights so don't be put off when traders try to tell you otherwise.