how to combat condensation

Hi My wife and I purchased our first caravan a 2005 swift challenger 490 in June last year, and we have had a great season with lots of short weekend breaks. When we bought the van it was in great condition and had no signs of damp and none could be found despite a thorough probing with a damp meter. We store the van on a local farm and after getting a call from the farmer to say he could hear a beeping noise coming from the van, I paid it a visit and was horrified to find a black mildew all over the front of the top cupboards, there was also a build up of moisture on the skylight flyscreen in the bathroom. I can only assume this has been caused by condensation and am hoping that I have caught it early enough to prevent any long term damage. Thankfully the farmer is very friendly and offered us a spot in his barn for the rest of the winter which I am hoping will put a stop to the problem for this winter. Can anyone give me any advice on the best way to avoid this happening again.

Steve