RS-422 should have high noise immunity already.
If you can build yourself, UART->Opto->ADM488 or ADM489 would work and be simple. Keep the opto on the UART side where there is only one data line.
I look on Vishay for opto couplers at the UART side, The rise/fall time for the fastest device (VOT205AT) is slow 3uSec, for data rate of 115200 the bit is 8.6uSec, so i will remain only with 8.6uSec-3-3=2.6uSec which looks very small value, Do you familiar with high speed couples ?