We use a 700w oil filled radiator for background heating, silent, keeps the chill off even at -20 in the Alps, £19.99 from Screwfix.
We then have a 2kw fan heater that we use for “main” heating when we are in the van during the day. £10 from Tesco.
The best thing we have is an electric blanket, leave it on all night, toasty & warm even on the lowest setting, only 150W so can use it on any hookup, £19.99 from Aldi