I'm with Klaus on this, twisting the wires alone will serve no purpose and twisting RS232 signals will make things worse. RS232 also requires a common ground between the two ends.
In a situation where noise is high, such as being parallel to Ac power wires, the best solution is to use two twisted pairs, one for transmit and one for receive, this is typically implemented as RS422 standard. It is differential so it cancels noise emission and pick-up and the receiver end works on the principle of the difference between wires rather than the difference to ground so it allows SOME difference in ground voltage between the two ends. There are still limits to ground voltage differences so be careful with power supplies at each end.
There are many RS422 drivers/receivers on the market, even ones with them both combined in an 8-pin package and because the signals are at 5V level, they do not need the extra charge pump capacitors that a MAX232 requires.
Brian.