There’s a D-Tap plug on both the battery and the ONsite Aero V-Mount, which should I plug the ONsite D-Tap to AC Power Supply into?

Plug your power supply directly into the plate, not into battery – this way if your D-Tap battery is depleted you can slide the V-Mount D-Tap battery off, replace with another D-Tap battery and your AC Power Supply is still plugged in ready to power and charge devices.