I'm not talking about measuring the terminal voltage of the battery. What I mean is using the ground lead as a current shunt by connecting one meter lead to the negative battery terminal and the other to where the ground lead connects to the vehicle chassis. The voltage measured across that shunt is proportional to the amount of current being drawn from the battery.

It's the same principle as measuring the voltage across a fuse, except it ends up measuring the draw for the whole car, not just one circuit.