I wonder if the Onan would settle down a bit with a higher load, assuming it too had only the 600W resistive load. Certainly I would expect that the relatively high frequency noise (possibly from the brushes?) would diminish; the 3rd harmonic might not change too much.
If the Onan is powering the converter and it's doing much other than loafing along, that may possibly be contributing somewhat to the distortion in the waveform.
All of these so far look perfectly acceptable for pretty much any use I can think of. The Onan might be a little less desirable for sensitive radio equipment with poorly isolated power supplies due to the high frequency noise.
