Since your rooting the tab, that would be the best way to go. You should check in to buying an OTG adapter that allows you to have a usb and micro sd slot since you will be giving up your main slot just for charging. Your only problem is if you want to keep it running while the truck isn't. Is there an option to have the tab shut down instead of going into sleep mode?
Being in sleep mode will eventually drain your battery. In your case a momentarily switch wont help you, you'll need a toggle switch instead.
My plan was to keep it stock and solder two wires to the power switch and run that to a relay and timer. So when vehicle starts it will activate the timer for lets say 3 seconds, so it will be as if your pressing the power button on the tab. The reason other reason why I want to open the rear of the Nexus is that the line out and charging slot will be in the way of the bezel frame, so it will have to be moved to make a nice finish.
At this moment I'm trying to trace the wires from my steering wheel buttons so I can solder them to the up and down buttons of the tablet.
Reason why I'm keeping it stock is because I've jailbroken my ipods and iphones b4 but I find it becomes a pain in the *** when you want to update the filmware. Also feels it doesn't run right when rooted.