we get the same in energy efficient homes now too, and is one of the biggest problems in the building industry. Condensation is turning water vapour back into liquid form as it cools again. So the trick is finding what is causing it.
While condensation is fairly obvious on a cold hard surface such as windows or metallic objects, it will also occur on materials such
as the seats, bedding and canvas, even plaster in houses. Anything which has a colder surface temp than the air in contact with it. There is a temperature differential figure but I can't recall off the top of my head what it is. But this is when the mould spores start, so correction is needed before mould takes a hold.
If over night, then most likely cause is people breathing. So knock it off.
Other causes can be cooking in the van, showers and washing machines. All increase the level or moisture in the air by heating water, and if not exhausted will later cool and condense.
The third reason can be the location - warm to hot days and cold nights can also cause the effect.
These will all be more pronounced in an 18 van more than the others due to only one end being canvas.
while the suggestion of leaving a vent open is an aid, you really need two open preferably at opposite ends. This will allow cross flow ventilation and the air pressures will draw the old air out and the vapour with it. The door vent is not really sufficient for this purpose alone, but does help. (its also there as any gas leak will sink to the lowest point (door well) and escape through that vent).
The suggestion above of having the heater on also works as the air is warmer and thus the vapour doesn't cool sufficiently to condense. It can end up stuffy and muggy instead though, as the vapour is still there, but just warmed. (humidity)
But ventilation is always the answer. You'll need to play around a little to find what works best for your comfort, and to suit the causes though.
