Excessive humidity is a common challenge for vandwellers. It can cause mold/mildew, damage to electronics from water condensate, discomfort, wood expansion, and diminution of insulative properties of items like sleeping bags.
Common causes of high humidity / condensation include:
Humidity due to constant habitation can be reduced with increased ventilation, as with roof vent, assuming exterior humidity is lower than that of the interior.
Window vents can be used to crack windows without allowing rain in.
A “sacrificial window” may work as an unpowered option.4)
Relocation to a drier climate sounds like a severe response, but snowbirds often follow comfortable weather.
Water vapor is often released during cooking, most visibly during a boil. Methods to reduce the production of water vapor include:
Products containing calcium carbide (DampRid, etc) or silica gel (Eva-Dry, etc) may help control excessive moisture, particularly in very small or confined areas. Some silica gel units can be “refreshed” by plugging them into a 110v socket whenever the color indicator advises. This gently heats the gel and drives off the moisture, so do this somewhere other than your camper!8)
Eva-Dry units are named after the cubic feet of space they are designed to handle. Example: the E-500 is made for 500 cubic feet, approximately 8x8x8'. Units larger than 500 are peltier or some other non-chemical technology.
In some cases an electric dehumidifier may be necessary. Bitty recommends the Eva-dry Edv-1100.9) Some users drain the receptacle into a sink or other drain.10) Eva-Dry confirms the 12v adapter for this model passes 12v and not 9v;11) this should make it easier to wire from house power.
Peltier devices are power-hungry and run constantly; the dehumidifier might best be used as an opportunity load.
In some very dry regions humidity may be too low (=<20%)for comfort. Evaporative coolers or ultrasonic misters may help improve the interior humidity.