I would think the best solution is to invest in a car heater that doesn't require the engine to run. They exist, but require electrical power from the battery to power the fan. But according to Webasto, they're really designed to heat your cold car for a short period of time, not keep it warm for long periods. Except that the fan will eventually drain the battery I see no reasons why you can't run a Webasto system for hours and hours. Not actually having tried this - except in a military vehicle years ago - I would ask someone with actual practical experience. Not cheap, though.