Hi,
I was looking up Energystar.gov website. I know that de-humidifiers use a huge amount of power. This is a link to a average high efficiency dehumidifier, it consumes about 1.8 kilowatts of power per liter of water recovered. That is more energy than can be stored in a 12 volt battery to just capture 1 liter of water! And to survive the whole winter, you would need to use 3-4 amps at 120 volts for a couple of hours per day, depending on how much air is circulating into the RV during each day.
I don't think you would find something that is 12 VDC to run your dehumidifier.
lCickyFred.