You have two batteries in that C?
The Fleetwood C I know starts the Onan 4000 off the house battery.
Check your voltage at the solenoid of the generator's starter. Should be AT LEAST 12.5V
You are checking the big heavy Red wire on the solenoid. One will be 0V, that's the wrong one ;)
Do this WITHOUT the start pressed.
You could also jump across the solenoid if these voltages are good. From big red to big red. But remember, BIG power, sparks, motor may turn over, etc. If you're not comfortable with that, don't ;)