Make sure there is ventilation in your campervan to allow the air you exhale to escape. Open the window(s) a little at night, use the optional mosquito nets if your are worried about bugs coming in. If your campervan comes with a rear awning (they all do, except for the Dream Sleeper Mini range) use that to create extra ventilation.
See how easy it is to set up the rear awning.
Also please make sure there are no damp clothes or wet towels in your camper, as this creates extra moisture.