I lived in my 71 Westy in Syracuse in 20 deg. typical day winter-  I added
a roof vent (the popup kind that's about 8" round); before that I popped
the roof up- this allows a fresh air updraft if you crack the window next
to the stove or heater inside van. (vents the gases better).
 Best is a gas heater that draws outside air; I used it before bed, then
shut it off (uses electricity for fan)- and turned it on before crawling
out of the sleeping bag.  A large heavy curtain hung a few inches just
beyond the bed limits the heated zone to the rear live in area when you
fold up the bed.    MB in NJ