Whilst the Bus is sat on the drive, I use 1 of these, on a low setting 24/7, just to keep the chills off.
Delight in a perfectly heated room with De'Longhi's compact oil filled radiator
www.delonghi.com
Used it for the last 5 winters
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Open all the drawers/ cupboards etc.
Our camper is kept fully stocked with a mattress / quilt / sleeping bag / pillows / rugs - so this 500w oil filled rad, does the trick.
Never had any issues with frozen taps / bottles / pipes.
Don't know what it costs to run, but it deffo keeps damp at bay