I found this while searching for the power consumption of a modern vehicle.
http://www.autohausaz.com/volkswagen...l-systems.html
It says a modern VW but they have failed to indicate how modern.
It says that the ignition consumes 6A. I am assuming that is the car ecu and the injectors and other stuff just to make the engine run.
When everything is on full blast, it consumes 77A which would be difficult to manage with deep cycle batteries.