Some of the older KY series and some others have a quirky setup, the output voltage is directly related to the input voltage, 12 volts in, 120 volts out. The excess gets clipped, but it has no ability to boost low voltage so if it’s 9 volts in, it would be only 90 volts out.
As a safety measure if the output voltage is too low it shuts the engine down as soon as you release the start button so you don’t burn stuff up.
The easy test is to continue hold the start button down after it starts and see if it keeps running. The starter button bypasses all the safety’s while it’s engaged.
If that works check the engine battery and / or try starting it with the coach engine running, the alternator will fool the genset into thinking the battery is fully charged.
