I doubt that a separate External power supply is needed. I have a monitor, from the same ebay dealer, I think, with that option but it was only needed if the voltage being measured was too low to operate the device. No need when it has the 12 Volt battery connected.
If you then think of the shunt as being part of the unit (in my 20 amp unit, the shunt is inside the case), the whole thing has only three wires to connect to the outside world. Two straight to the battery and the other to the RV ground. Note that you have to disconnect the battery from ground so that all current from ground back to the battery goes through the shunt where it can be measured. I hope your unit has sufficiently heavy connectors to the shunt to handle the wires you want to use to battery negative and ground. If not, you will have to downsize the wire to a gauge that it can accommodate without breaking.
Of course you want it to measure charging current from all sources, just as it is shown in the diagram. It can't show the state of charge without measuring the charging current. No extra wiring to accomplish that - the engine, converter and solar must already be connected to ground and battery positive.
my monitor setup with just 3 wires coming out. Two heavy for the current and one light to power the unit and let it measure voltage.
2004 E350 Adventurer (Canadian) 20 footer - Alberta, Canada
No TV + 100W solar = no generator needed