When the truck is plugged in is the electrical system designed to use the cable power to feed the auxiliary systems, or does it always use the battery and continuously top it off once you hit your desired capacity?
Designed for that? No, but it is in the nature of things. Lets say your battery is charge to 380V. Let's ay you have set your charger to charge at X amps (DC, not wall amps), The charger will regulate its output voltage to maintain X amps. That voltage will have to be greater than 380 V, say 385V. The charger and battery are both connected to the DC bus. So are a couple of other things such as the traction motor inverters, the compressor motor inverters, the DC/DC converter and the heater(s).. As the charger is at 385V and the battery at 380V most of the current to the compressor, when it is on, would flow from the charger but some may come from the battery too.
The other loads, such as the headlights, are powered from the 12 V batteries. If they are discharged sufficiently the DC/DC converter is turned on to energize them and, as it is on the DC bus, is the same as the compressor. Much of its current will come from the charger but some may come from the high voltage battery too.
If the charger is set at a level that passes more power to the vehicle than the sum of all the auxilliary consumption then all the power to the auxiliary loads will come from the charger with any left over going to the battery.
