I suspect the RCDs are a red herring.

(Dons computer engineer hat) Early Hammond organs using tonewheels driven by a synchronous motor were susceptible to changes in mains frequency, which could be a problem on a festival site relying on generator power. However, modern electronic keyboards get their pitch from a crystal oscillator, which is utterly stable - think quartz crystal clocks. They are also quite tolerant to mains voltage drop: as Jack suggested, if this was the problem it is likely to have affected the entire hall.

On your return home, did you have to reset the tuning to its normal setting?