When my 1997 Onan 5KW BGM (only 74 hours!!)wouldn't run after lifting the start switch it turned-out to be a broken ignition rotor. It functions like a distributor rotor in a car firing circuit. The rotor is cheap plastic and is mounted between the engine and generator on the main shaft. It had split and was no longer making and breaking contact correctly.
An Answer.com tech coached me thru to the cause. Good guy. I had to make a temp support table, pull the gen onto a temp work table, split it and replace the $9 POS rotor. then reasemble it all and install. The tech's help ran $48 including a $10 tip. Worth every penny with local RV labor $90-100/hr.
Just my experience. good luck, these things can be a real pain.
