Urgent problem zero electric power solved… I’m an idiot

That's likely the issue.
As there are typically 2 feeds from the + post.
One to the starter, one to the fuse box.

And then one from the alternator to the battery. But OP is running a dual system.

Not sure why you'd need to remove the cable from the terminal to swap a battery.

But in any case. If this is the issue. I'd use some red electrical tape and "label" that wire +.

 

Need to trace that wire to be sure (hard to tell for sure on the picture) but it sure looks like that's the wire coming from the fuse box stud.

 

He could do that with the resistance check I laid out above.

 

FWIW, OP you got lucky here.
You didn't blow any fuses. There's only a few ways something like this can happen and not blow at least 1 fuse.

 

That wouldn't tell the difference between a misconnected terminal and a normal situation due to no power going to that stud though because you would still see a path to ground either way.
We need to physically trace the wire at this point, it shouldn't be difficult as it's literally 12 inches of wire from the stud to the battery.

 

So disconnecting the wire from the battery terminal AND from the fuse box, then check resistance wouldn't work?

Check from one end of the wire to the other.

 

I agree with the two feeds to the positive.

My question back to the OP would be did he change anything when he swapped the battery out in the first place? I have the same terminals and I would not need to disconnect any leads to swap batteries.

 

I agree. If he removed a a wire. Then the issue is likely the wire he removed.
But might as well check to see where the fuse box wire goes.

 

Ok so here's what the stock setup looks like, the smaller wire from the positive battery terminal goes down into the bottom of the fuse box to the main fuse.
The output of the main fuse feeds the rest of the fuses and the stud, the cable from the stud goes down to the alternator.

So I was incorrect in my initial assesment.

 

I've always found that when something operated correctly before I touched it and no longer works there's about a 99.9% chance I screwed something up.

 

Just to be clear this is the wire we are concerned with.

 

The wire from the stud should route to the engine harness that goes down to the alternator.

 

And here is where we need to know how he's setup the DC-DC charger.

We've completely isolated the fuse box of any + connections.
And somehow we've managed to make the fuse box a "ground".

I wish I was there with the truck. This is interesting.
And the issue is probably a simple fix. But the diag is "a process".

 

Again, that’s why I said to voltage drop test those individual cables.
It would isolate which side, and which cable.

then run THAT down and check for:
Bad crimp (or soldering)
Bad cable ie cut, severed or otherwise not grounded

you aren’t drop testing it with a load. You’re drop testing it from the battery and running it down.

if it’s not 12v on that cable, then that cable needs to be pulled or bad ground. If it’s 12v from battery neg post to different part of the cable it could be a bad crimp or solder and so forth

 

How do you do a voltage drop test NOT under load?

 

12.6 volts is the load in this case. Your just using a multimeter…

your going across the battery post to the connector. The battery in this case is the load.

 

Ok. Are you SURE the wire that comes in from the bottom, doesn't go to the top?
You have "other stuff" blocking the view.

 

Yea I traced them, the engine harness with the alternator wire goes to the stud on top and the wire from the battery goes into the bottom of the box.

If you unlatch the little slide with the engine harness you can slide it up and see.

 

If the battery had been going directly to ground, something would've melted or the battery would've exploded. The fuse box is not connected. I agree that the one wire going to the negative terminal may likely be the culprit. I also agree that it should be marked with red electrical tape to make things more clear in the future if this is the case.

 

I’d be curious to see what the voltage reading is at the alternator.
Engine off.

It should read battery voltage.

But then again, OP is running dual batteries.
And no idea how he setup charging.