Damp Smell

Hello all,

Firstly, my apologies if this is in the wrong place or already been covered, however,

We purchased a Lunar Quasar 2005 Caravan and whenever it has been standing for any period of time, it develops a damp smell. We have checked everywhere but unable to find anything inside the caravan that is wet or damp.

Does anybody know what could be causing this and how to eradicate it? When the van is parked up and not used we usually keep the blinds to the windows down but have recently kept the roof blinds open in the hope of allowing light in. 

Any thoughts or opinions on this would be much appreciated. Many thanks.