Got it all wired up and it works great! Only problem I ran into was that the firmware I had installed on the Arduino did not have PS_DEFAULT_OFF defined. Therefore when I plugged in the power supply the RAMPS got 12V. Since the RAMPS was already powered up I couldn't connect the RPi to the Arduino/Ramps... It would try to connect then say "Machine State: Closed". Not sure if this is default behavior or if something else is going on.
At any rate, putting "#define PS_DEFAULT_OFF" into my configuration.h file and uploading to the MEGA fixed the problem. 12V stays off until I send the command "M80" to turn it on. "M81" turns it back off.
At any rate, putting "#define PS_DEFAULT_OFF" into my configuration.h file and uploading to the MEGA fixed the problem. 12V stays off until I send the command "M80" to turn it on. "M81" turns it back off.