The subject of dehumidifiers comes up each year, with divided views on their effectiveness.
If the caravan was totally sealed, they may provide some use, but as caravans have a lot of fixed ventilation, all you are doing is continually attempting to dry ever changing air in the van.
Unless your van has a leak, there should be no need for a dehumidifer, and if it does have a leak, that should be fixed first.
Unless you can guarantee looking at the crystal versions very regularly, you may find that they produce so much water that they overflow and can cause damage to whatever surface they are resting on.
As always, the choice is yours.
If you feel some kind of benefit from using one then just be aware of the possible pitfalls.
Personally I have never used any kind of dehumidifier and have never had a problem with damp.