You need to check the error code from onan gen to get an idea what may be wrong..
From the Start/Stop switch on the gen (not inside coach), press and hold the stop for about 3 sec, release. count the number of blinks on the switch. there will be 2 sets. after the first set of blinks, there will be a brief pause, followed by more blinks (ie 3-6) that combination will determine any error code generated by the onan.
if this MAY be fuel related, you can prime the system by holding the stop switch on the Gen for 30 sec. you should here fuel pumped thru the gen. try after that see what the gen does.
assuming your tank has more than 1/4 of fuel. sometime the fuel sensor on the tank goes bad, that would also shut down the gen. have had that a coupe of times. sensors erratic reading hard to see, except a beep. that was enough to shut down the gen.