You might want to check out http://groups.yahoo.com/group/rolandrs/
, maybe someone there could help you.
It doesn't appear that the RS series has any Mellotron string waveforms (seems to have some Mellotron choir though, strange it would have one and not the other...). If you want to try making your own, take a fairly realistic string patch, set the attack and release times to zero so it's instant-on, instant-off, and EQ or filter it to take out the highs and lows, giving it that low-fi quality. Add a bit of detuning too (or Analogue Feel, not sure if the RS has that parameter or not).