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.