We're slowly getting there...........

If the battery is 'floating' the shield of the cable gets connected to the frame (metal) of the bus. The exact location is probably not critical (apart from connecting it to the exhaust pipe - the cable would melt!). Note that your system would probably work equally as well without a shield (I'm assuming the bus has diesel engine as it has no ignition system to cause interference). RS485 is quite tolerant and in your application you could wire with hookup wire and no termination and it would most likely work - not that I suggest this!!!

Since you're running power through your 0V wire - beware of voltage drops across it as this becomes a common mode voltage to the 75176 - keep the max voltage drop below about 2volts and you'll be ok.