Can I add a twist to your subject.
I totally agree with the answers so far in terms of long term storage, but there can be case to use a dehumidifier in some more specific circumstances.
The humidity in caravans is raised when people use the caravan, what we breath out is very humid, and as an example over night a adult can easily exhale as much as 100mL of water as vapour. When we use the open flamed cooker and oven, for every gram of gas used the flame produces about a gram of water vapour, and then of course there's the steam from boiling kettles and pans. Room seal appliances like the space, water heater and fridge will also produce water vapour but it is ducted directly outside and does not contribute to the interior of thecaravan.
So during use there is moisture build up in soft furnishings and the woodwork of a caravan - Oh and of course if the caravan has a water ingress problem that too will contribute.
All this is frequently obvious when the windows mist up at night. So during or immediately after periods of usage a dehumidifier may help to extract the excess moisture, but there is no benefit in using one after a couple of days in storage.
Obviously it won't do any harm (except to your bank balance) to leave one in long term, but there is no effective benefit
The best solution to control long storage humidity is ensuring the caravan is well ventilated, with soft cushions lifted or even removed and doors and lockers left open to aid free air circulation.
And getting any water leaks repaired!