How we can possibly transfer a USART signal(3.3v) over 12 meter cable without any driver?
In a harsh environment you may really need a driver to get reliable results. Large motors may influence the voltages on the cable and cause transmission errors.
So, if your boss is truely an unmoving mountain:

use a low transmission speed,
check for errors (USART, CRC),
re-send (or discard) data if errors occur,
thouroughly test before you deliver;

Also, if you do this, don't tell your clients it uses RS-232; RS232 specifies the physical layer too... Using 0 and 3V3 makes it 'not RS-232'.