If you have any type of bed in your vehicle, mattress ventilation is very important to keep mold and mildew from forming. Ensure that the space under your bed is well ventilated. If that's not a possibility, you'll need turn over your mattress regularly so that its underside can dry out. An alternative solution is a plastic waterproof mattress cover, which may decrease comfort.
If you have fixed bed over an unheated space (such as an underbed "garage") this is especially important as condensation is likely to occur due to the temperature difference. The surface on which your mattress rests should allow air flow with slats or holes; placing the mattress directly on a solid surface increases the risk of moisture problems. See the ventilation page for more on keeping your living space adequately ventilated.
You can also add an underlay for ventilation. This permeable layer elevates the mattress and allows air to flow underneath.
If you have a convertible bed or unattached bed that gets moved daily, moisture may not be a problem if the underside is regularly exposed to air. However, many people with convertible beds get tired of changing them back and forth and just leave the bed in bed mode and work around it.