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Glad you made it. Actually kinda fun trying to brainstorm what was going wrong with your truck. What did they say "failure is not an option".

I think it's transmission harness issue. The rain got into something.

I would re-seat all the connectors related to the Allison. Behind the dash, the one at the left front of the engine under the cab, and the ones at the transmission. Look for chafing on the harness.

DNS is do not shift (the TCM that is, not you)

P0880 Disappearance of power supply control unit during operation engine P2685 Low voltage in the circuit HSD3 P2812 Open circuit solenoid PCS6 Incl. DNS, SOL OFF...

Codes mostly related to voltage issues in TCM. Do you have a way to google the codes? I think the WTEC is inhibiting crank because it feels really sick.

How about pull codes on transmission? Press up and down key at the same time and report back.

Voltage too low!! <- Retract that, voltage is OK with alternator not outputting.

Is "MODE" lit on the trans controller?

Now you know why Uncle excessed the truck. Nobody had days to spend chasing this issue. Should be fine now, you can operate it normally.

If the LBCD checks out, next downstream is the power distribution panel, which was soldered with RoHS solder and is subject to intermittent connections.

The device under the spare is called the LBCD (Load Battery Control Device). Search that term under the FMTV forum will bring up what little is on the internet about that device. A post by suprman has a schematic.

Bypass it, it is intended to protect the circuits in the event that the batteries are hooked up backwards. I think you can "double lug" the cables on one side of the device.

Look for a device with heavy cables going in and out. It has been known to have intermittent connection issues.

Next, check for power in and out of reverse polarity device under the spare tire.

"Dash stays day", what does this mean? Check the solenoid by the batteries. Control power to it? Power out of it?