I presume that 'true RMS' means what is says, which would mean that it would give an RMS figures measured over all components (including DC ones) - which is probably not what matters in terms of RCD operation.
That was my point - and, as I've just written, I would suspect that (in the face of messy waveforms) a 'true RMS' one might well "match an RCD's response" less than most!
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