Use this for a quick reference and you'll see that volume is much much more than twice as much square footage:
For my bus, a 40 foot bus, that will have 6 double paned windows, the windshield of course, everything else sheeted over, 2 inches of closed cell spray foam in the ceiling and walls, 3/4" foam board on the floor, an not accounting for dead spaces like cabinets and barriers like the bathroom and bedroom walls, it comes out to the following:
Width: 7.5 feet
Length: 35 feet
Height: 6.1 feet
Insulation condition: normal
- you are still in a metal box, which can be a cooker or a freezer
Temperature change: 80 degrees F
- assuming at -20F outside worst case, and 60F inside
I'm looking at getting a Webasto diesel fired heater that will do a little over 20kBTU on its max setting and am still concerned that it may be too cold to endure. Since it says you're in Chicago, I imagine it gets cold there in the winter. If you stay in an apartment, with other apartments above, below and on each side all running their heaters, it's absolutely nothing like being in the bus surrounded by the raw air. Just like metal conducts electricity, it conducts heat - from where it's hot to where it's not. You'll freeze in the winter unless you camp directly in front of that 600w heater.
Like you said though, real life will show the way. Pack lots of blankets and a fire extinguisher.