Without seeing it, it is very difficult to know exactly what it is, but almost always black spots are usually mould. The fact it is spots also suggests it's probably the results of condensation.
The fact you have found moisture traps full, shows the atmosphere was laden with moisture, and that does not mean you necessary have a leak, it's just nature.
Warm air can carry more moisture than cold air, which is why we often heat things up to dry them, the warmed air drags moisture from the washing and then convects it away.- Result... faster drying.
Condensation occurs when warm moist air is cooled and it has to get rid of some of its moisture. This often happens when the air comes into contact with a cold surface which appears as condensation.
In a caravan it is very easy to have areas that are quickly heated due to the sunshine, yet areas in shadow can be much cooler, especially at this time of year.
The best solution to tackle condensation is good ventilation, such as leaving cupboard and bed lockers open to allow free movement of air.
Dehumidifiers can make a difference when you put a caravan into storage. When the caravan has been in use it will have absorbed some additional moisture becasue we like to keep them warn (which raises the amount of moisture things han hold), and we breathe, and use the cooker and oven, boiled kettle's etc, all of which increases the level of humidity inside the caravan, and in soft furnishings and exposed wood.
When we put it into storage and it cools down, all that extra moisture is released into the air and condenses in the cooler areas. Dehumidifies can help to collect that excess moisture, but only until the level in the caravan fall to match the ambient conditions. After that point the continued use of dehumidifiers offers no further practical benefit, because the air inside the caravan is slowly but continually exchanged with new fresh (and moisture laden) air from outside.
All the continued use of a dehumidifier does is to try and collect all the moisture in the atmosphere, and will make no practical change to the conditions inside the caravan.
I suggest you clean the mould with either a propriety anti mould cleaner or weak bleach solution, and make sure you allow free air movement inside the caravan.
Edited to correct spellings