Overnight condensation

Hello.  Not sure if asked elsewhere, but grateful for any advice.  Have 2009 Sterling Europa which I use with family (2 adults, 2 children & collie dog!). During autumn and spring use, we use blown air heater on very low overnight due to cold ambient outside. We keep Heki roof vent x2 open but still wake up with huge amounts of condensation affecting every window in the van.  I do not see others on site as badly affected.  Van serviced and no damp. No wet towels etc inside.  Any ideas where I am going wrong?Thanks in anticipation.