Pointers The air that contains the most moisture is the warmest air ...the warmest air is always uppermost - so bleeding a proportion of it out at the highest spot can be done .
insulation - air condenses on cold surfaces...insulation will stop this ..but you cant insulate the windows....so will condense there instead.
bleeding air out ......well that's your heating gone.
Dehumidifiers - well forget any form of fridge compressor type electric one as it will run your battery down in hours.
There are small 12 volt ones that are a piezoelectric, don't use much power ..i bought one once and put it to the test in a steamy bathroom........right after i had run the bath ,.despite hardly being able to to see 6 feet with the steam ...it only milked out of it about 50cc in 1 hour - still said 100 percent humidity on a gauge i had in there .
Most of these piezoelectric ones have a mains transformer that is 230 volts / 12 volts ...so you'll be buying one with a car socket on it .
https://www.google.co.uk/search?q=piezo ... humidifierThen find somewhere high up under the roof of the van to install it.
note. non of them work very well below 15 degrees c. and my failed test above .
BTW ...the very nature of dry air, means that the moment you open a door ..the air will be instantly replaced with whatever moisture content there is outside...so all efforts will be lost .
all the best.mark