I just had somewhat the same problem. My Onan 8000 diesel would not start. I would hold down the stop button to heat glow plug then the start button. Nothing would happen other than I would hear the fuel pump working. I cleaned everything looked at everything. My RV tech told me to start the engine let it run for 30 min or so then try and start the gen.. Sure enough the gen started.
I also was plugged in but found out the coach batteries were no good. The volts would read 13.5+/- so I thought all was ok. Took batteries out and had them tested the volts were good but the amps were not there to do much of anything. I learned its the amps that to the work.
Just another note the gen also started with the motor running and the coach batteries disconnected. Get batteries checked
Itasca Meridian SE 36g
Road Master Tow Dolly