The goal of any ESD design is to shunt the unwanted current back to the source or to earth. If an ESD event happens on RS485 cables, the best thing to do would be to shunt it to earth which in most designs is through the PCB ground, then chassis ground.
Because RS485 is differential, if there are TVS diodes to ground on both lines then then any common mode noise will be subtracted out. The diagram below shows a configuration for TVS diodes to ground.
Source: https://www.onsemi.com/pub/Collateral/AND8229-D.PDF
In noisy environments, it may be best to use shielded RS485.