Yeah, even one of the larger leisure batteries contains at most one kilowatt hour. That's completely useless for heating up any space for any length of time.
If you can target the heat where it really needs to be, like those dashboard things or electric blankets / heated jackets / gloves / etc you can get away with far lower wattages and have a far longer runtime, but still very much not great. I'm not sure that can be applied to either of your intended applications.