My tool of choice for this kind of trouble shot is a test light, I have 3. 2 I bought one I made (i like it best) I will tell you how to make one. If you buy buy a good one, (One of mine is)
I took a license plate lamp socket, takes a common blade type lamp, inserted said lamp. on one of the two leads I put a simple alligator clip. The other lead I soldered and taped on a good 3 feet of wire, and a 2nd clip on the end of that, If I ever get "Fancy" I'll put a probe on the other end.
Starting at the battery since you know it's good, Touch one clip to each terminal (POSITIVE/Negative) should see the light. GOOD.
Now move the NEGATIVE (note it don't matter which clip is negative, only that you move the negative clip. if you use an old fashion incandescent lamp) to the chassis, Still see the light, GOOD.
Now move out along the positive wire with the other clip. If you need to move the negative clip make sure you still have a good ground by touching the positive clip to the last known good spot. When you no longer see the light.. The bad spot is between light and dark.
Suspect is your isolator solenoid, In my case it turned out to be the control switch Same test light, and a partner, confirmed that and simple repairs were done by me ASAP.
I did need the service manual for my Battery control system though to know where to apply the test lamp.
Home is where I park it!