I figure if rpms were a big deal, they would specify that in the manual.
Engine RPM's and the frequency of the generated voltage are directly related. The frequency that the generator produces is based on the number of revolutions the motor turns the rotor per second.
60 revolutions per second times 60 seconds = revolutions per minute (RPM)
60 times 60 = 3600.
I suspect that the reason that Generac does not specify RPM is because measuring the AC frequency is also measuring RPM and most folks who work on their generators have a multimeter that will show the frequency but may not have a tachometer. In fact, either measurement provides the same information (if one does the math in the post by Canada_Guy above).
The manual never once mentions rpms. Just specifies volts and hz.
Actually, a tachometer is a far superior instrument to use because it does not depend on the generator's ability to function to measure engine RPM. I have and use both instruments, by the way.