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Most of the time this is a battery issue. When was the last time the batteries were serviced and load tested? You might want to start there first. Be sure that all the cables are clean and tight, especially the lead to the chassis.
If you have a small digital multimeter you can check to see if you have at least 12 VDC TO the start solenoid on the genset. Let me know what you find. About $20 or so for one at most hardware stores.
Just checking back in to see if you were able to resolve this issue or do you still need some additional input from me here?