With our portable home dehumidifier in our back main bedroom we have to run it 24/7 to get the proper effect from it..at 60%. 260w. No condensation at all ..in our current 60+yrs renovated wooden board house.
So don't know what effect a 12v will have in a 6m bus but maybe if run over night it will help. They can be noisy once they start the actual process, not just the fan going.. They are a mini fridge concept, hence leaving ours on instead of turning it off and on.