Recommend me a heater for van.

A USB heater? No. You need a proper Webasto/Eberspacher/Chinese copy of the aforementioned which will require work to function properly. Otherwise wrap up warmer or double up your sleeping bag.

Basically what they said ^^^^

USB and 12v doesn't have the grunt to heat a van for any length of time without killing your vehicle battery or the leisure battery. I looked into using a cab warmer that runs off the 12v socket for my Dispatch and, despite it being rated at 200w or something, it would have been a noisy, inefficient way to heat the back (even with insulation) compared to spending a couple of hundred quid on a diesel heater.

The Chinese copies can work if you either fit them properly, or are happy leaving the standalone ones outside and running a warm air inlet into your load area. If I was using my van for weekends at this time of year (I'm in Sweden) I'd be doing that, but the 4 or 8kW ones they advertise are way overkill for the van I have. 2kW would be a better fit for the size of load area, but are more expensive.

Honestly, you might be better off getting some 25mm Armourflex fitted under the ply lining and then just making a bed with a thicker duvet. If you are doing that though, get a vent/extraction fan fitted as well.

Forgot to say, YouTuber has a LOT of videos for comparisons between the various Chinese brands. The consensus is apparently to choose one that has the most generic components so that you can get spares/upgrades.

This is an example of the sort of thing that would stay outside and need a long hot air hose to your cab/load area: https://www.amazon.se/-/en/Portable-0-16-0-52L-Adjustable-15-20m2-Intelligent/dp/B0CB3VD6C3/

This one would need you to chop a hole in the floor of your van, then seal up and gasket to the max, but is probably more what you want for the size of van:
https://www.amazon.se/-/en/dp/B0CFQNS2R6/

Are you staying anywhere with electric hook up? Quite a few cheap options available down that route. I'll run a cable out the back door, the door rubber seal being thick enough it allows the doors to shut without damaging the cable.

ACLS fitted a diesel heater for me which keeps my van nice and toasty.

They came to me and fitted it in about half a day.

You do need a descent leisure battery set up to run one though.

Indeed hookup = very cheap. We have a small oil filled radiator when we're on site, saves the noise and blowy of our Webatso.

If not, you need a proper heater.

I would also say that insulation is not that expensive and can make a huge difference. Our High Top barely needs any heat unless it's below 3*c or so to be comfy, and we sleep in pretty chilly (-7 so far) without overnight heat. Because it's insulated.

Yeah my sister swears by an oil filled radiator rather than using her diesel heater in her camper. I've not tried one in mine yet but you usually only need to make it not cold rather than warm.

How about an infrared heater?

You need a proper Webasto/Eberspacher/Chinese copy of the aforementioned which will require work to function properly. Otherwise wrap up warmer or double up your sleeping bag.

Autotherm.

Basically a copy of the Webasto at a third of the price without the will I/won't I die with the Chinese copies.

Autoterm heaters (and Planer - same brand) as a brand are owned by a Russian company called Advers. They have opened a factory in Latvia but are still largely using parts from their factory in Russia. So that will be a hard no for me.  Reminds me I need to book my Webasto fitting.

Propex/Eberspacher/Wesbasto or a noisier Chinese copy.

The only electric option that would be practical might be an electric blanket, a 12v car version/heated blanket would be most efficient, if they exist.

I went with a Lavaner 2kw for my van, to avoid the Russian option or death by Chinese tat. Plumbed into the fuel tank. Pretty straightforward but a bit faffy fabricating a mount under the van for it and trying to find the optimal spot to bring the pipes through.