Dehumidification is a topic that has cropped up many times within the touring caravan community. Unfortunately motorhomes and touring caravans are not sealed boxes and are designed to have some fixed ventilation. You must never obstruct this ventilation as it is essential for when we use them.
The causes of humidity, damp and condensation are often not fully understood. Briefly, air can carry an amount of moisture, how much depends on the temperature of the air, when it's warm it can carry more than when it's cold, consequently, when moist warm air is cooled, it has to get rid of some of its moisture so it condeses out as water droplets. That's what causes rain, and condensation on cold surfaces.
Conversely as air is warmed up it can absorb more moisture, which is why we use hot air for drying things. Similaremperature effects also apply to many materials used in caravans.
There are a number of activities that create extra moisture inside, cooking and making hot drinks, but do not underestimate humans and any animals. One of the most significant sources of warm moist air is what we all breathe out.
When we use caravans, we tend to raise the temperature of the internal air and of course the fittings. This makes all these things capable of absorbing some of the moisture we create so during use the vehicles moisture content will rise.
When we put it into storage, the vehicle will tend to cool down, and some of the absorbed moisture will be released , but as the air is also cooler it can't absorb it so it appears as condensation.
The vehicle will only release its moisture until the relative levels of humidity between construction materials and the air are matched. Once equalibrium has been reached, no further transference will occur.
The upshot of this is that there may be some benefit to using a dehumidifier for a few days after the vehicle has been used, but longer term it will offer no practical benefit over maintaining good ventilation. It helps to open all spaces to allow full ventilation.
If you have a persistent damp problem, it would be wise to check for any structural leaks and to resolve those rather than treating the symptoms.