There are specialized RF voltmeters that can do a true rms voltage measurement, but is that the parameter you want for this measurement? A common parameter for modulated RF waveforms is PEP = "peak envelope power", which is the mean power associated with the voltage at the maximum excursion of the voltage. A peak-to-peak responding voltmeter (which can be built easily with two capacitors and two diodes) is more appropriate for that measurement.