What is 2nd breaker box for

[2by4]

UGGGGGGGGG

Your converter is wired the same as mine was. . Take a look at this thread where I disconnected mine and trace the wires back. https://gdrvowners.com/forum/solar/5...-don-t-mess-up Sorry for the long read..
Do not touch 2a and 2b. You can leave them without any issue. There are two black (hot) wires connected to the breaker (wire nut). I left one attached (not the one from the converter) as I could not determine where it went. Then disconnected the ground (green) and negative return (white) and leave the rest in place just in case of failure. Be sure to wire nut and tape the wires you disconnected. Doing this would make it easy to reconnect if needed. No issues so far.
Hope this helps

Try and remember Murphy and the Kiss law.... Kip it simple.......Stu... I have learned this lesson many times here Mostly due to Howard, Jim, and Rob

Hoping to help
Keith

In that case I am confused where would the wire from Multiplus/battery can connect as both 2a/b and 4a/b are still there. There is no available terminal to connect to. I get the disconnect convert from AC part. Now I’m thinking where does DC can connect to.

[Yoda]

In that case I am confused where would the wire from Multiplus/battery can connect as both 2a/b and 4a/b are still there. There is no available terminal to connect to. I get the disconnect convert from AC part. Now I’m thinking where does DC can connect to.

The Multiplus 12V connects to the battery bank and only the battery bank - usually with 2/0 cables and fuse, It draws power and delivers change/converter power through the same connections. The trailer power also comes from the battery bank through the trailer hot buss bar that is also connected directly to the battery (second edit and also connect to the hot side (red wire) distribution panel). The two circuits are separate and the trailer circuit is protected by the OEM re-settable breaker. Not sure how to explain it better - sorry. The experts will be along shortly. I suggest re-reading the Multiplus install manual from Victron.
Hope this helps
Keith

On edit
The 120V shore power should go to the Multiplus input and the 120V output then goes to the 120V side of the distribution panel. The 12V side connects to the battery bank. Leave everything else the same except for simply disconnecting the OEM converter.. It is basically plug and play. Just saying how mine worked.