Hello Brian,
I am not a solicitor; so only use this information as a guide. You should seek professional legal advice if you wish to pursue any action.
As a purchaser you afforded certain legal protections through the statutory rights relating to the Sale of Goods Acts and Trades Description regulations. (see DTI web sites)
http://www.dti.gov.uk/consumers/fact-sheets/page24700.html
http://www.dti.gov.uk/consumers/fact-sheets/page24702.html
In a brief nutshell - a trader/retailer/dealer (excluding private sales) is obliged to supply goods fit for purpose and matching the description offered. This is relatively easy in the case of new items because of published specifications but it also applies to second hand items, however the law does recognise that a second-hand item will be subject to wear and tear commensurate with its previous history.
A trader is legally obliged to ensure that any statement they make about the product is as accurate as reasonably possible, but the customer is also obliged to ensure the product meets their own requirements by asking all relevant questions before the sale is made. Hence the phrase: Caveat Emptor or Buyer Beware.
In practice this means that if the dealer had described the caravan as being free from damp, then you would have a case if damp was evident from the time of sale, but if you did not ask about damp, and it was not brought up in writing or conversation during the negotiations then the dealer has not miss represented the product.
Clearly it is better to have documentary evidence, but as this would be a civil action the courts make judgments on the balance of probability, and circumstantial evidence may be considered.
For example if a dealer is member of a trade organisation that suggests or applies certain standards for its members to adopt, then if the action of the dealer transgresses any of the organisations protocols this could be viewed as failing to follow accepted standards.
If the dealer is a member of a trade organisation you might have success if you contact them.