Is a dehumidifier necessary?

No way! Your caravan has lots of necessary vents to the outside and if you use any sort of dehumidifier - mains powered or crystals - all you will do is draw in moist air and attempt to dry out the big wide world.

It's far better to make sure air can circulate inside the van by opening cupboards and either removing bunks or standing them on edge away from outside walls.

Strange advice for a dealership to give anyone.  All the many vans on dealership forecourts won't have dehumidifiers in them.

Exactly as above. We empty bedding, towels etc out of the van. All doors and lockers are opened and seats/mattresses moved away from the caravan sides so air can circulate.

The really big must is a full drain down of the water system including the toilet, leaving taps and valves open. [unless you have a how water heating system with anti-freeze, as at home]

Colin

Don't waste your money!  just follow the advice already given.  My caravan is at my house and when the weather is fine I just open the door for an hour or so to let it air.  It's open as I type!

Sounds like someone wants to sell you a dehumidifier!

Pointless given the ventilator holes in a caravan. Dried air inside simply sucks moisture in from outside. Without mains power you’d have to use a chemical dehumidifier which probably is not wonderful anyway. Keep all ventilators clear, maybe remove soft furnishings. 

Thank you knowledgeable people. Good thing I checked.

The atmosphere is dynamic, despite your best efforts it will constantly be balancing temperature and humidity in and out the van. Best you can do is remove all materials which could potentially store water like bedding, leave all cupboards as empty as possible and doors open. Maintain a good through flow of fresh air inside the van and work with nature. Don’t try to seal off air vents, they are needed for air flow and your safety. Dehumidifiers can help to remove water already trapped inside the fabric of the damp van but you never dry out the atmosphere. Best solution is to maintain a dry van in the first place and eliminate any possible ingress of rainwater causing the damp issue.