You are right about the car being in park. This is one of Eco Idle preconditions (just as the accelerator pedal is unpressed and the vehicle's engines are not really could (in this case, the truck will use another strategy to control the engines to heat it))
About the fuel consumption, Eco Idle...
Well, I have been using this truck in a while and comparing Eco Idle Enabled and Disabled. The basic difference between Eco Idle and Normal Idle is that Eco Idle uses a higher RPM to charge the battery AND it charges more battery percentage. While Normal Idle charges up to 49%, the Eco Idle...