Carl
Do you mean a DE-humidifier. A humidifier introduces moisture into the air, which is the opposite to what you're probably trying to achieve.
I've heard of people using de-humidifiers, but this would be an endless exercise. With all the vents in the caravan the air is constantly being replaced. I think that it's something like once every hour the air is replaced. This is done because of venting for the gas, and it prevents damp air from being 'sealed in'. Therefore all that you are doing is drying out your garden and the world as a whole. As soon as the air is de-humidified, it's replaced in the caravan with fresh humidified air. The only effective way of doing it is to completely seal the caravan, then run the dehumidifier until all traces of moisture are removed.
It's the same with silica gel that some people use. It's OK in a sealed cupboard, but in the caravan living space is a complete waste of time. And in the case of silica gel, you'd need dozens of kgs of the stuff in order for it to be effective.