When I lived in a van for a few years I started with a Chinese diesel heater. Whilst effective at blasting heat into the space it was often a case of being too cold or overwhelmingly hot. I also couldn't use it at night as the pump was disgustingly loud. I transitioned to ultimately using a sub zero mummy style sleeping bag for the nights and just using the diesel heater on and off for when I was awake and this was more than adequate even when the temp dropped below zero.

After my first van blew up and I had to get a different one I got one that had a similar style heater but it ran on LPG with an external hook up to refill the tank at petrol stations. This one I set up with a couple of vents fed with ducting and was far superior to its diesel counterpart. It was quiet enough that I could leave the temp sensor on its lowest setting and coupled with a decent thick duvet and sleeping in clothes I was always warm enough and when it did come on in the nights it never really disturbed my sleep.

I miss the van life to be honest, houses are overrated as fuck but I got a girlfriend who was not into the idea of vans.