This wouldn't be as simple as it should be. There's a lot involved with the camera feeds.
Basically, the car has to be on for the cameras to work. In addition, they disable >5MPH. If you can circumvent those, it's just a matter of an adapter to send the video feed over BT/WiFi.
I'd be concerned about the 12V drain, though. The SLA wasn't made to handle loads like that, and may not even last 8 hours. (12Ah SLA ~150Wh)
Edit: For clarity, I assume the camera feeds were meant for video capture while the car is parked/off (no DC-DC converter). The speed switch may not be an issue when it's parked, but the circuitry definitely adds complexity.