Dethleffs have been independently contacted  regarding moisture in the garage area and received this reply :
It is not possible to assess the situation described on your vehicle from a distance. However, this topic comes up in the cold season. Ultimately, condensation is caused by insufficient ventilation of the affected areas. The warm air, saturated with moisture, circulates through the vehicle and settles in the colder areas for example in the rear garage. This causes condensation to form in these areas. The only remedy here is the right heating and ventilation strategy. It is also important that the air can circulate in the affected areas and is not hindered by massive loading.
Kind Regards
Your Customer Service Team
Kundendienst – Customer Service
Tel.: 07562 – 987–0
Dethleffs GmbH & Co. KG
Arist-Dethleffs-Str. 12
88316 Isny
As you can see this is not a new issue with the Factory .
This only ever seems to be a winter problem particularly with  vans left on storage sites without hookup , all closed up for longer than normal. Even our modern double glazed houses end up with moisture/condensation in a kitchen (heat) on a cold winters day and opening a window,   slightly , solves the problem ( ventilation ). I know we cant leave the garage / locker doors open ,on unattended vans,  but many have internal access so perhaps we could ensure air can move about freely .