RS232 serial output: confusion with wiring
There are ways of "stealing" power from an RS232 port, but 30mA is pushing it, and you may not even get 12V.  So it's likely you'll need a battery or other power supply between 12V and Ground to power the sensor.

You can make an RS232 connection with just 3 wires - Rx Tx and Ground.  You need a cable with the Rx and Tx swapped, and the Ground wire connected at each end.

If the two ends are reluctant to talk, Google "rs232 three wire handshake looping".  You need to loop back some output pins of the RS232 port to fool the port into thinking that it is fully connected to something at the other end.