I am a carpet cleaner and leave hoses and fluids in vehicle overnight (machine always in house)
I use a mains Greenhouse Fan Heater Model MP1 1250watts
Findlay, Irvine Ltd, Penecuik, near Edinburgh
Has a slow speed fan and a wide range thermostatic temperature control which I usually set on 2 (0-12)
which is very low temperature - somewhere not much above freezing but not checked it.
Keeps frost off inside of van windows (open cab/load area in mine)