Have a look at the attachment.
If I understand your set-up correctly, I would connect as per the top sketch if I were using a simple test meter.
One thing to bare in mind when using newer high impedance digital meters, they can respond to high frequency noise (radio transmissions etc) picked up on the long single wire.
In many ways, an older moving coil type meter is sometimes better in a situation like this.
If I wanted to use an oscilloscope of some kind, I would connect it as per the lower sketch.
I would not use the scope probes, I would just use a coax cable connected normally with a BNC plug to the scope input.
At the remote end, I would connect the centre of the coax to the device under test.
Back at the scope, I would connect the outer of the coax to the test earth spike.
So, that is how I would go about it.
What I would do or change after having done this... I have no idea!
JimB