[DonDavis]

For parking with electrical access...
I remember seeing a heater placed inline with the engine coolant system with a built-in recirc pump. Leave the vehicle heater set to "Hi", the heater element warms the water and the small recirc pump just maintains coolant motion, at a moderate pace, and heated water is circulated throughout the entire coolant system.

In the morning, disconnect the power cables and stow away.
Engine starts immediately with warm air blowing from the vents.