Why condensation happens
The acrylic used in these windows is slightly hygroscopic (i.e. attracts water), so in extreme conditions, moisture will permeate the material. For this reason these windows are not sealed units, so any moisture can escape between the panes. Normally no moisture builds up, but it will happen if it is cool inside, warm and moist inside and the caravan becomes warm very quickly. In this case temperatures and humidity do not have time to balance and condensation forms.
How to clear condensation
If your window has plugged ventilation holes, remove the plugs. Park your caravan slightly open and towards the sun, this will draw the moisture out. You can speed this up slightly by putting a warm, dry towel on the outside pane. During this process the moisture may appear to go away and then come back, this is because there is still moisture in the acrylic material that needs to be drawn out.
It is important that there is adequate ventilation in the caravan to prevent the build up of moist air inside the vehicle. In particular, make sure that the ventilation provided by the manufacturer is not blocked in any way. Provide extra ventilation by leaving sky lights open and any partition doors open. Do not leave water in kettles, pans or containers. Do not do the washing up inside the van last thing at night. Do not have the caravan too warm at night. Increase the ventilation in wet or humid weather.
Condensation should not cause too many problems for most caravan owners, but if parts of the windows are remaining damp for a long time, then mould may form or discolouration to the acrylic may become visible.