As Swift have stated you will always get condensation when air hits a surface of different temperature. Switching off the heating system on a cold night and it is amazing/disappointing how rapidly the van cools off, but not if you look at it realistically.
Although you may feel your van is 'insulated' in reality it is not that efficient. Although the walls and roof are insulated, and the floor is a thick piece of chipboard there are holes all over the place. Each skylight has permanent openings and even the windows are not really the same as your domestic double glazing. They have 2 panels but no vacuum, or gas filling. Many UK vans also have poorly installed refrigerators, and the air can easily enter from around the unit. Your heater also is fitted to the floor, but there is no guarantee is is fully sealed around the base as the hole is precut. You may also have cold air coming up around your shower waste. Sorry, but that is the way they make 'em.
Your fixed bed headboard is located directly under a window in all probability, so any cold air entering there will hit the warm wood, and condense.
Unfortunately, your best answer is to keep the heating running in very cold weather, so the van continually heats any cold air entering. Of course, if you use gas your addition to the carbon foorprint will not endear you to the Greens.