My 6.5 KW Onan pooped out while running, & it's the AC side that's down.

The likeliest cause is a bad wire or connection, or dirty/worn brushes. I'm about to tear into it myself (it isn't rocket science) & I've ordered a factory service manual, but some experience-based cautions, or info. would be helpful.

Has anyone here been inside one of these things, & have experience they might share?

I worked on my '85 Onan 6.5. The generator head has a shield over the brushes. This shield has a latch on the bottom like an old suitcase latch. Unlock the latch and you can check out the brushes and the wiring. I had a broken wire coming off of a terminal. You can pull the brush holders out with a 5/16 nut driver and clean them.