On an HGJAB unit, if it is running, it is producing power.
First double check the breakers on the genset. Shut them off, and turn them back on.
Next, if you have an onboard receptacle that you need to plug into for genset power, make sure the shore cord is plugged in.
If this does not take care of it, your problem is most likely in the transfer switch,
Locate the transfer switch, and use the wiring diagram on the cover to determine which leads are the input from the genset, and which leads are the output to the coach.
With the genset running, test the input an output leads. If you have good voltage coming in from the genset, but no output to the coach, the transfer switch is bad.
If you do not have good voltage coming in from the genset, the problem will be in the junction box located near the genset. Open it up and check the wiring connections.