I can only speculate, since I don't own a 230rs, so not sure if there's something special with the wiring.
a cut off switch is normally on the positive side of the battery, so you can connect your inverter positive cable to it, as long as the wire leading into the switch from the battery is at least as big as the wire leading to the inverter.
a shunt is normally on the negative side of the battery, and is the "last" connection between everything else and the battery's negative terminal. you would want to connect your inverter's negative cable to the load side of the shunt, along with all the other trailer's connection. this will allow the shunt to measure the inverter usage. again, same caution applies that the cable between battery and shunt needs to be at least as big as the cable between shunt and inverter.
