Onan Microquiet 4000 making grinding noise, won't start.

OP, I'm not sure what kind of RV the Onan is in. But if it's an A, B or C with its own engine, try starting the chassis engine first and letting it run. Then try starting the Onan. Sometimes the higher voltage from the alternator is what the starter needs to spin thing fast enough to get the engine to start. (And if you already said you tried this, apologies.)

Jim, "I'm retired. Go around me."