You can't connect logic-level optocouplers directly to RS232 level, you need to use some kind of level translators, either IC or discrete circuits. O.K. there's an option of driving TxD with 0/5V level and hope the receiver will accept it. But that's not what I would do in the said industrial enviroment if noise immunity is an objective.
You also need to design a parasitic power supply utilizing TxD and possibly additional signals. If you buy the nice Chinese (or whatsoever) RS232 isolator, you get everything ready-made.
By the way, why do you say you don't use RTS/CTS pins? Using additional RS232 lines as auxilary power supply can considerably ease the isolator operation by providing a permanent positive voltage.