Using Zener diode for circuit protection on RS-485 network
I wouldn't trust a standard zener to protect a line running outside. Can you show the circuit? Are you sure it doesn't mean a TVS diode anyway?

You could use a single (say 6-10V) TVS on the lines as you mention.
A more expensive but more robust solution would be to use a three stage protector like this (or make your own)
It combines a gas discharge tube with a series impedance and a TVS. You get the benefits of the gas discharge (high current capability, high breakdown voltage, slow) with the TVS (fast, low clamping voltage)