Small genies are usually 220 volts especially the dual voltage ones as it's both 110 coils in seriesThe power from the cheapo gennies can be clean, in the sense that the output is a clean sinewave, but voltage regulation is relatively poor. Still doesn't explain 2-30V a.c. and 400-700Hz, though. Should be more like 250-270V, at 50Hz-ish.
I looked at your manual and instantly saw the problem, its the ubiquitous Chinese nasty 2 stroke geny, they can't maintain any steady rpm and are no good for running sensitive electronics. Forget about getting any kind of 'clean' power out of it.