Ventilation,Ventilation,Ventilation.
It’s a learning curve and yes we still suffer some days with it.
If you’re cooking or shower in your van make sure you crack a window or have your fan on extraction to get rid of excess steam and moisture.
We installed a shower fan in our shower area and purchased driver and passenger window security vents (these have bug mesh over the vent grills as well).
Our biggest and one of the best improvements we installed was an extra large skylight it has a trickle vent bug screen and blackout screen.
Insulation plays a big part if the outside temperature is colder than the inside of your van the moisture will settle on any cold surfaces like windows and metal that has no insulation behind it or covering it.
Need more info on what we installed feel free to message Adventures in Walt.