Sorry to see that you are having troubles.
Let's hit a couple of things, mostly common sense.
Is the fuel tank of your RV more than 1/4 full?
(The generator gets its fuel from a tap that is at least 1/4 above empty.)
Did you prime the gen set before trying to start it?
(For most Onan's, depress the Stop button until the green light comes on--same as when the genset is running OK. Depressing the stop button primes the fuel line so that it is full and has not drained back into the tank.)
Try doing that, first, and then try to start the gen set when the green light comes on.
Are the RV house batteries fully charged?
(Most of the Onan's can be kantankerous if the batteries are not fully charged. Start the RV engine and let it run for two or three minutes--and then try to start the gen set while the engine is still running.)
The RV America resident RV tech guru, Dave, will most likely have some other good ideas.