Thanks everyone for your advice. I just heard back from Victron via email. I think I understand their response but it would be helpful if anyone else could confirm I'm interpreting correctly. I had asked them whether the neutral wire needs to be branched, so that line 1 and line 2 have separate neutral wires into the breaker box and potentially even separate neutral bus bars. Sounds like the answer is "No" -- to simply run line 2 direct to the breaker box without its own separate neutral/ground, and then run line 1 through the inverter along with neutral and ground.
Victron's exact response:
For proper current measurement and control, as well a preventing multiple return paths, it is advised to run the ground an N through the Multi without branching off to the L2 line first:
1) Only one side of the distribution panel would be powered by the inverter when not plugged in. Also no 240V inverter loads are possible with just one Multi.
2) If you only have single-phase 120V input available, make sure it is on the L1 line through the Multi. If you try to run it to the L2, the open input relay on the Multi will prevent a complete circuit. You would want it this way for charging anyway.
Does that all make sense? In the image he sent, it looks like the power source is coming from a breaker box on the right rather than a shore power cord, but I assume the answer would be the same in a shore power or generator sourced setup.
The other thing that could be troubling is his point #2. Let's say I'm using a 30A to 50A adapter cord to to plug into 30A shore power or a generator... my understanding is the the adapter provides (a smaller amount of) current into both legs of the 50A cord. So I don't think his point about the open input relay preventing a complete circuit would apply in that case, since both L1 and L2 would be receiving power.
Again, would appreciate if others have comments. This seems to be the simplest possible setup for me. It doesn't require installing a separate breaker box. I'll simply need a little junction box so I can splice and extend the L1, N, & G to the inverter and back.
Thanks,
Joe
