In the van-life world, heating is very frequently based on a fuel-powered heater such as the Webasto line from Cummins: <https://www.cummins.com/na/sales-and-service/webasto-heating...>.

There's also typically ventilation to reduce moisture/condensation build-up in cold climates. Even with heat-exchange air circulation, the incoming air does require additional heating.

So, yes, this is a legitimate concern.