If you know there is one thing calling “truck cab air conditioner”, it will not hard for u to know how the truck drivers keep their trucks or sleeping cab cold.

It is a specially-designed air conditioning system for cabs in trailers, trucks, tractors and construction machinery. The truck cab air conditioners are usually driven by battery - DC 12V/24V battery engine, which makes the cab cool while the engine of the trucks are off. Therefore, it is also called “Parking Truck Air Conditioner”.
The parking truck air conditioners can be divided into 2 types: wall back mounted and rooftop mounted ones. Except installing on the truck, the DC 12V/24V air conditioners can be also used in RVs, Vans and other special vehicles.
Here is one of these kinds units manufactuered by Guchen Industry:

The truck sleeping cab air conditioner’s small in size, but high in cooling efficiency.