Using differential line drivers for digital signals
RS485/RS422 hardware layer is based on differential line transceivers. The 5V is chosen, because of high speed transfer. Note that a 12V level signal with same slew rate needs more time to "reach" the HI or LO signal level, so small voltage signal is needed for high data frequency. The protocol layer is something totally different story. 


Typically a RS485 communication can go up to 1.2km @ 9600 bps,  twisted pair, with a conductors diameter >=0.64 mm (AWG 22)- courtesy of Siemens.


You won't find differential signals at high voltage, except some current loop, also used for data communication over distance. This is now obsolete in industry and replaced with RS485, because at time there were no such RS485 transceivers, I guess. The current loop was made with an optocoupler and a current mirror.








simulate this circuit &ndash; Schematic created using CircuitLab


With an isolated RS485 comm. you would need to supply the RS485 transceivers with a galvanically isolated, floating PSU, like DC/DC converter 5V/5V. You have to bias the signal pairs with Vcc and GND potential through bias resistors, and of course the termination resistor at both ends. In such way, the PSUs are equalized to common potential voltage. There is no need to connect the earth on them, and of course it has to be avoided to connect the GNDs of both PSUs throuh a third conductor like shield. The shield is earthed on one end to sink the environment interference. 




At each side you put a bias and termination resistors.







simulate this circuit

Link to ready made transceiver with bias and terminating resistors.