You may want to view my video on the topic of RMS voltage. The first part of the video talks about what RMS means and shows some examples of calculating RMS for sinewaves. Then I talk about square waves (that are balanced around ground), and then I talk about digital pulses, like PWM pulses which have their "low" value equal to ground, and some high value equal to Vp. The video shows that the RMS value of a PWM signal such as this is Vp * sqrt(duty_cycle). The discussion of PWM waveforms begins at about 6:45 in the video.