You can see how I used a $17, 8-position fuse block in my Overlander here
This is a shunt
. It is important that it is in the ground
(-) wire from the battery(ies). You can measure total current into or out of the batteries with the wiring configuration shown in the post. At 50 amps the shunt has only 50 millivolts across it, so there's negligible loss of voltage with it in the circuit. The problem with the old Airstream control panels was that the mechanical voltmeter was calibrated to 100 amps, plus and minus, so you couldn't see much movement when you were drawing less than 5 amps. Most of the time you're boon docking you're interested in small continuous current draw, eg, the radio at 0.5 amps. You just can't read that with the old voltmeter. Use a new digital voltmeter and you can read out 0.01 amps through the shunt.