Been busy today with work
, chasing down tools and teaching myself to "weld" aluminum.
The reason they likely have the dedicated cable from the battery negative to the inverter is because the inverter is the biggest amp-hog on your trailer. What I didn't see was a positive cable coming directly off the battery, going to the inverter. If they used a dedicated negative cable for the inverter, they should have also used a dedicated positive cable.
What I would do is this:
1. Figure out the gauge of the cables (both negative and positive) going to the inverter, chassis, solar charge controller and the main distribution panel (what you're calling the converter in the video - it's both a converter and a distribution panel for your lesser 12v items).
2. Remove ALL cables going to your negative battery terminals.
3. Get a new negative cable that is at least as big as the biggest cable from step 1. A cable at least one gauge larger is better. Get a cable long enough so you can run it into the trailer and out of the elements. The longer you have to run it, the bigger the gauge needs to be.
4. Run the new negative cable from the negative battery terminal (either one, it doesn't matter) to the battery side of your shunt.
5. Connect ALL the cables that were disconnected in step 2 to the load side of your shunt.
It's likely that your shunt can't hold all the ring terminals for all the cables. The bolt may not be that long, and the diameter of the shunt bolt may not be the same size as all the ring terminals. It looks like the stud on your battery is 5/16" (hard to tell). My shunt uses a 3/8" bolt. If that's the case then you may want to add a four post (or more) bus bar that uses 5/16" studs. Connect the load side of the shunt to the new bus bar. That's how I wired mine up.