Chinese diesel heater? Reliable and safe, uses a very small amount of fuel, but does need a well charged car battery to start up.
We have one in the camper, after a few hiccups with power supply it works great. Look up Eberspacher, they’re fitted to truck cabs, boats etc. Eberspacher are very expensive, but the system is simple, the cheaper versions run very well.
Otherwise, something like this Aladdin blue Flame below. There are some good paraffin heaters about, Ruby and Zibro I think are Korean makes, designed for domestic use.