I am just back in from wiring it as I thought in my own question. In my case I have wired all of the negative sides, including solar and the RENOGY DC-DC -ve to a -ve BUS and wired from there to the P-. The B- is wired directly to BATT -ve and from BATT -ve to vehicle chassis.
The shunt is reporting the solar panel charge current (and any disharge) in the DC Home app OK albeit that it varies a bit from solar input amps showing in the DC HOME app DC-DC 30 device. However, I suppose it gives me a SOC reading although not entirely confident how accurate it is possibly because I can't trust how balanced my Renogy battery cells are.
For £50 cost in my case, the shunt works, but I can see why folk go for the pricier Victron. You get what you pay for in life I suppose.
On a side note, anyone else have Renogy DC app and does it annoyingly ask you to continually save name of any devices you have connected?
Now all I have to do is fit new JBD BMS boards to the 2 x Renogy 50aH LIFE04 batteries where the cells (4S) keep going out of balance.
If I ever fit out another campervan...it will be Fogstar and Victron