Exactly, the dreamers in the van are feeling 0g, so they're dreaming in 0g as well. However, it's still a dream so when you reach the dream-within-a-dream level

Exactly, the dreamers in the van are feeling 0g, so they're dreaming in 0g as well. However, it's still a dream so when you reach the dream-within-a-dream level, the brain probably defaults to what it's more used to.