If you have it tapped into an ignition controlled source (parking lights, taillight), are you saying that the unit will stay on using the internal battery when the ignition is turned off?
This sounds like the better approach. It should have enough stored internal power to plan your trip before arousing the beast. Once started, the bike power source would take over and also recharge the internal battery. At a pit stop, the internal battery would take over when the ignition is off. Lastly, if you somehow forget to turn the unit off, it will simply run down the internal battery rather than the Roc battery. Is this a correct assumption?