If you can only get to the 75mV signal via the meter in the cab, you are going to have to use some kind of isolation amplifier for your own safety and that of the equipment. You can't use 75mV to drive an LED in an optoisolator without a gain stage, and you are going to have to power that gain stage from your "cold" side of the barrier. Several companies make isolation amps for just this purpose. Analog devices for one. You could even do it yourself, but you really need a transformer with GOOD isolation - uise the transformer to power a circuit on the hot side, and return a signal to the low side via the opto interface.

Steve