Mark, ALL negative loads must pass through the shunt. Just putting the inverter load through the shunt will not see any other loads from the 12V
Here are two ways to do it. Not sure of your layout, but you likely will have to uncover the work area to get at the front left corner of the trailer interior where all the wiring converges, by removing or partially disassembling some furniture to get to everything easily and safely. It's not as intimidating as it might seem once you figure out the right screws to remove. This is not something to have a skinny kid crawling in there to hook up :-)
Option 1. Replace the two wires from the battery box with one new 2 AWG wire to the battery side of the shunt which should be mounted inside (not weather proof). (I presume your current wires are 4awg to the inverter and 6 awg to 12v
negative bus. You need to increase the size of the wire if you switch to just one, to carry the load of the 12V
and the Inverter at the same time.)
Then reuse the 4AWG wire, hooking it from the load side of the shunt to the inverter fuse, and reuse the 6AWG wire from load side of shunt to negative bus.
This method makes it easy to also add a dedicated battery ON/OFF switch to use when in storage. Just put the switch somewhere inside on the new 2 AWG wire between the battery and the shunt.
Option 2 (Avoids having to pull new wire through the floor from the batteries). Move the 4AWG inverter wire from the batteries that currently goes to the inverter fuse, to the battery side of the shunt. Also move the 6AWG 12v house wire that currently goes to the negative bus, also to the battery side of the shunt.
Then run two new wires from the load side of shunt: a 4AWG wire to inverter fuse and a 6AWG to the 12v neg bus.
Either way you will need to make or purchase new wires with appropriate connectors.
If neither of those are an option, then you *could* figure out a place in the box (if it fits) that you could have both neg wires go through the shunt, but as mentioned it needs to be weatherproof. Also you will need to go through the floor penetration to run the telephone wire from the shunt to the monitor. Unless you tried to put the monitor outside too. My vote is to it right the first time and use option 1 or 2 above.