I agree with Raywood , there's enough ventilation though the gas drop outs and I doubt the widows be left ajar would make any difference to the level of damp in a caravan.
Maybe some of those water trap thingy's would help a bit. But I think they try to remove the water content out of the whole atmosphere not just the caravan.
I just take the soft furnishing out of our 'van and store them in the house over winter. That not only protects them it allows the air to circulate around the interior and help eliminate areas that could hold water against the wallboards.
Anyway a good few 'vans are ventilated by the howling gale around the fridge housing!