Powering SN75176 for RS485/UART communications over long distances
RS485 requires a ground. See here.   

"Proper wiring[6]: RS-485 specifies differential transmission, which requires two signal wires in addition to a ground wire (commonly a 24AWG twisted pair) to transmit the signal. The two signal wires carry signals opposite in polarity, and greatly reduce the problems of radiated EMI and EMI pickup. 

In the SN75176 datasheet you would not be able to guarantee the common mode signal levels if there was no earth/ground connection between the Rx/Tx pairs. If you are going to span more than a few hundred feet you may also find you earth potential will vary by several volts, so it's best to provide isolation to avoid this.

You may be better using a +/- 20 or 60 mA current loop providing your data transfer (baud rate) rate is modest.