I agree with Oscar. Wires need to be identified and labeled.
If you look at Fig 3-4 Section 3-7, The drawing shows 7 wires leaving G1.
Four of them are the main focus here for now. The wires they have marked as
M4 - is neutral, M2- is ground, M3 - is a phase conductor, M1 - is also a phase conductor.
The ground should be easily identified by measuring resistance to the chassis. (should read zero)
The drawing shows the neutral and ground bonded at the terminal strip. I doubt it's bonded inside the generator? Maybe so? Your ohm meter should prove this. The phase conductors are shown connected also. Then the drawing shows this terminal feeding cb1 with an optional cb2 with 120v only potential.
That does not match your nameplate yet it matches the RV way of use so I am also cautious to advise connection. Ac voltage needs to be measured between any combination of conductors. There should be wires elsewhere leaving G1 going to the starter relay (k1), a wire to grnd and a wire to a blocking diode. (K1 energizes a winding in the generator making it a temporary starter motor.) Not real concerned about that right now because you can start it. Later on we can check for dc voltage after the blocking diode.
I now realize after pissin out the martinis and beer that my Onan 4.0 is a contractors model and doesn't fit this conversation. I know absolutely zero about this battery isolation gizmo. So, I am probably not much help here.