Thanks everyone for your help!
As I was thinking about this, I realized that since I bought the bus in October, I've only started it a few times, just to move it in or out of the driveway. It probably never ran enough to charge batteries. In addition to being in the single digits, the batteries were probably kind of low, then I ran the coolant heater for an hour or so.
It sounds like the coolant heater, when running off of the batteries, should be used to supplement the heat and not really to keep it warm for starting.