Winter Heating!

Went out to check on my caravan today sitting in her paddock alongside my mates van.  She looked very sad in her winter mode but, I was cheered by the fact that at least her batteries well up to charge as my mate lets be connect to his mains outlet in the stables.

That got me thinking, he I know, keeps his heating on very low.  Not enough to warm the van but enough to keep it from freezing inside.

Has anyone got any ideas on that practise. Is it good practise, or is it dangerous re fire or wearing out the heater etc.  If it is acceptable practise, does anyone know how much I should offer him for electricity used.  He doesen't want paying for the electricity used to keep battery charged, but I don't want to take advantage, if the cost is significant.