When you refer to the bar inside the panel you mean the metal (probably copper or copper alloy) that the main + cable from the battery
connects and has many small screws where wires or breakers attach?
And if you measure the voltage at different points on that same, solid, copper bar you get different readings? Unless the metal itself has degraded due to stray current corrosion
or something similar that just doesn't make sense to me at all.
Ignoring this issue for the moment, have you checked the ground side of the circuits? You checked all connections including removing the battery cables
the terminals and cable connections? What about the groundn side: main ground to the engine
block or negative bus bar?
Another test, if all the above check out disconnect the main + and - wires at the panel and battery and check resistance on each end to end. Maybe do the same for that mysterious bar in the panel.