You used to be able to get heaters that sat under the sump overnight.
I saw one once in the shed of an empty cottage that was for sale. Thing a bit like a Davy lamp, with a fine mesh cowl surrounding the wick (presumably to reduce fire/explosion risk from ignited petrol vapour), fuelled by kerosene.
I was tempted to knick it (I'd bet the eventual purchasers just chucked it out) but conscience prevailed.
I'm not sure its such a great idea though. Seems possible condensation from the combustion would cause more electrical grief than the heating solved
I have a cheap wee hotplate designed to keep coffee/tea warm, which would probably do the job, but likewise no convenient access to power, which of course is where the combustion-based heaters score.
IIRC some British Army Landrovers were fitted with diesel-powered pre-heater burners, maybe by Eberspracher?
Since it never gets below zero here (except rarely in the mountains) I don't really need one, though I suppose oil-preheating would still be a good thing in the "not -summer".
It'd mean, for example, that I could go back to using straight 40W engine oil all year round..