You need a sensitive current meter and place it in the line from the battery
. A digital multimeter on Amps would suffice.
Measure the current flow and disconnect things to find out what is drawing the current.
You can also place the meter across the shunt and measure on the millivolt range and then do the same thing. the higher the voltage the more current.
The shunt is usually in the negative ground line of the battery(s). It draws no current itself, just a very very small voltage drop proportional to the current going through it.
If the shunt has been bypassed with any other grounds (to the battery) it will not read them.