You say you are 12V with up to 4 12V batteries in parallel but have drawn them in series to give 48V.
What is the converter for, to give a stabilised 12V supply.
The negative of the converter is wrongly connected, it needs to go to the 12V negative bus bar so that the SmartShunt will measure the current going to the loads.
When you add solar the negative from the MPPT MUST also go the the negative bus bar.
Read the Multiplus 2 manual for required wire gauge, 6 gauge is much too thin. This also applies to the cable between the bus bars, shunt, isolator and batteries. If you follow the recommended twin cables to the inverter, then single cables of the same gauge go to each of the 2 initial batteries.
You have not shown fuses, do not forget them. The bus bars will need to be circa 500A rated to deal with the Multiplus load.