What symptoms caused the desire to test?
Couple of suggestions... disconnect the positive battery cable if you're trying to see if there is any potential between the battery negative and the disconnected negative cable. By putting the voltmeter in the circuit you are completing it and there will be voltage running through the voltmeter. You just woke up some modules when the VM was put in circuit.
For the brake test, leave the key out; the electric brake circuit is (should be) always hot and turning the key on should do nothing. Then check your brake pin voltage, but check it in reference to the 7-blade ground pin AND with a know good ground on the truck. Compare. Note that the brake controller is what varies this voltage and it won't be 12v with no complete trailer brake circuit and no brakes commanded.
You may have a bad ground in the trailer circuit. Check and clean the frame ground near the 7-blade.
You can also see if there is voltage to the 7-blade brake pin with the controller disconnected (again, testing with both grounds). Obviously there shouldn't be any. With the brake controller disconnected you can ohm the brake wire; but another better test would be to put a real load on the disconnected circuit while testing with a fused 12v source; something like a 2A bulb or the like.
I assume that you've tested the voltage (12v) to the controller, and the ability of that circuit to handle a load of at least several amps.
There's a short test procedure on this eTrailer page:
https://www.etrailer.com/question-53547.html
Also check and clean body and engine grounds; these trucks are susceptible to failing grounds and that causes all sorts of electrical issues.