Powdery mould ?

Do you leave anything at all in the wardrobe? It only takes the tiniest bit of damp, say on a pair of beach shoes, or a presumed dry, but actually not, beach towel to trigger such mould. Leave the wardrobe door open as well, to let air circulate. I’d investigate what might be under the wardrobe bottom as well.

Have you cleaned the 'wooden' surfaces with something like Pledge? It leaves a residue which seems to become mouldy.

Wipe it ( and other surfaces) down with white vinegar, should stop recurrence.

When in storage open all lockers and doors, let the air circulate.

I've had something similar in our Bailey Ranger. It's our first van so new for us. Ours was in close vicinity around the fridge area on the wood. Wiped down once and then went in 2/3 weeks later and still some, but not as bad. Last time wiped with Milton (I think it was). Not been in since to look.

Like you rest of caravan was spotless, clean and dry.

I used to get this on the outside of the interior cupboard doors.  Note, I used the past tense, so how did I stop it?  First of all it will go with a wipe down, but you want to stop it appearing at all.  I have been caravanning for over 30 years and first suffered the dusty mould about 5 years ago when I stored my caravan in an area surrounded by trees, in the shade almost all the time and protected from any wind.  I then changed my storage to a Gold Star site near the sea where there is plenty of air movement and sunshine.  In the first year I still got a little but since then I have not had a recurrence.  From this I conclude that air circulation is the key to ensuring this does happen.  Before storing for the winter I ensure all surfaces are clean and all vents clear of anything.  I open all cupboards and put the mattresses on their edges.  I visit every couple of weeks to open the door and roof light to give it an airing.  I was not able to check the caravan this winter from November to last week but when I went to check  it was free from any sign of the dreaded dusty mould.  So that is 3 winters free now.