Question regarding moisture absorbtion during periods of storage

Southwest123

I am relatively new to caravaning so this may be a daft question. I have just visited our caravan which is in outdoor storage - last visited in October. In side the caravan was a light "dust" over the interior woodwork. It looks like a build up of moisture has left a residual. I was able to wipe this off with a dry cloth. Is this normal?

I would add that the caravan did not feel damp. Perhaps I should buy something to absorb any moisture that may be present as part of the caravan becoming warm and cold.  Any advice appreciated.

Moderator Edit:

Thread title slightly amended to reflect that this isn't a question about storage location or costs.