For those considering a very expensive option:
@Braysaway mentions a small fan heater.
I use a 500w little block of a thing, sits between passenger seat and kitchen end (California, not MP) when travelling, and recently on-grid as I was towing my Sisters caravan and it did a surprising job at keeping the van warm. Total cost £18, uses about 3A so will run on french campsites without Pierre marching over threatening to disembowel anyone who trips his site's electricity and has proved to be amazingly rugged (drooped about 6 times now).
I have used it on tickover at night and been lovely and warm (and only use 4.5 tog duvet all year round).