Compute RMS value of non-stationary signal

I would like to know what is the most accurate way to compute a measured signal's fundamental RMS value as a function of time.

The following is true for the considered signal:

• signal represents a voltage
• sample rate: fs = 6 kHz
• capture time: t = 100 s
• fundamental frequency: f1 = 60 Hz
• signal is not a pure sinusoidal but has additional frequency content
• amplitude of the signals fundamental changes slowly over time