I like where this is going and helpful for others down the road.
In my case when I was checking I would put one test lead on main lug and the other one to a frame bolt or unpainted area.
I will do this test with the meter next time I look at it., If I dont get 12v when doing this test does it in fact mean I am having a ground issue (negative return path)?
Since the battery is the source of voltage, which supplies the current necessary to make the starter turn, it’s a good idea to make your measurements with reference to the battery posts themselves. NOT the clamp on lugs on the battery, the posts themselves.
All the starter motor cares about is getting voltage between the positive lug and the starter motor case, since the case is connected to the engine, and the engine is connected to the frame, and the frame is connected to the negative battery terminal via the clamp on the battery post.
If you measure voltage between the positive, post on the starter, and the case of the starter, this will tell you if the starter is seeing the voltage it needs to turn. And if the voltage drops when you turn the key to the start position, this indicates, you have an issue external to the starter. It is the magnetic effect of the current that flows through the starter that causes the starter armature to turn. If the voltage drops at the moment, you turn the key to the start position, then the proper current will not flow through the starter, and the starter won’t produce the normal amount of torque.
So the very important first measurement is to measure the voltage between the main lug on the starter and the starter case, when you turn the key to the start position, and when the starter should turn. Since you have had the starter checked by somebody that apparently knows how to check starters, and it seems to be good, chances are very high that you have a problem in the external circuit.
If you do, the next step would be to measure voltage between the negative terminal of the battery and the starter motor case. You should read 0 V at all times or very close to it. If you suddenly see the voltage rise significantly when you turn the key to the full start position, then you know you have an issue in the negative return path of the battery between the starter case and the battery negative post.
You can do the same thing on the positive side. If you measure voltage between the positive battery post and the main lug on the starter motor. You should always measure 0 V or very close to it. If that voltage jumps relatively high when you turn the key to the fully start position, then you have an issue between the battery positive post and the positive terminal on the starter.
Making these two kinds of measurements will certainly point you in the right direction as to solving the problem.
edit: Another very simple first test is to measure the voltage between the battery posts when you turn the key to the start position. The voltage should hold constant or close to it. If the voltage suddenly drops significantly, and the starter doesn’t turn, the problem is your battery.
