You will always get condensation in the van over winter. A wet brush head left in the van will create condensation overnight.
The best you can do is try to limit it by reducing the moisture you introduce inside during your working day.
Before we put the hose reel/s back in the van, we put a hoselok connector on the male inlet which is sealed with silicone. It stops water dripping into the van. The other end has a female hose stop. If the hose is wet then we put a towel underneath it to catch any drips.
We drain the pole hose down each time we put a pole into the van and shake off any excess water in the brush head. Don't leave wet towels/cloths in the van overnight.
Despite the precautions, we still get condensation on the van roof, but not dripping wet as some experience.
There are days when the air is laden with moisture and you will trap that in the back of your van when you finish at the end of the day. It can be seen on the inside of most car windows these days. We try to work with the van doors open when we can to allow it to dry out, and I often use a dry towel to dry the condensation off the roof in the morning.
Yes, those air extractors (Fettner vents) will help provided there is some wind to get them to work. However, they will also suck out any warmth you are trying to keep in the van during the night to stop your system freezing up.
You could also try a humidifier and see if that reduces the problem.