It's just basic physics - you've got very inside air, and unless you're holding it at temperature, the retained heat/cold in the seats/bedding/everything is fighting against you constantly. And to make matters worse, you open a door and lose most of whatever hot/cold air you tried to create. And then of course, it's small, so you're constantly getting cold/heat conducted in through the outside walls/glass, which are never more than a foot or two from you. Compare that to an apartment, where the outside walls are thick and heavy/dense, and you've got other warm/cool airboxes on 2-5 sides of your air-box. Vanlife seems romantic, but unless you're doing it somewhere where living in an actual tent would be tolerable, it isn't.