I'd start testing at the closest point to the radio and work my way back to the TIPM. Back probing (with a T needle or dedicated back probe) at the connector my preference over piercing the wire. Often the wire insulation is so brittle on these old trucks that it breaks a larger chunk off when you pierce it. Could buy dedicated back probes for your meter if the T needle seems too big. I like dedicated probes over T needles. If you don't have a volt ohm meter, you could use a test lamp with a probe instead.

I suspect you're going to find a crushed (or otherwise compromised) bundle of wire that happens to include the radio and 5th wheel feeds, or multiple corrosion pins points on a connector, assuming the radio and 5th wheel are on different circuits. A circuit diagram would help point out probable causes, like a shared ground point or power wires running in the same bundle. There are often opportunities to download the factory service manuals, and they contain the diagrams.

Good luck!

 

I finally had the time to tear the IPM apart.
There was no visible corrosion anywhere, but I cleaned all the connections and terminals, verified continuity from the radio fuse terminals to both the main hot lug and the output pin to the blue connector on back.
Cleaned all the connection plugs and liberally used electrolytic grease on all connections.
Cleaned all the fuse terminals and reassembled everything.
Cleaned the battery terminals.

Everything worked when I started the truck so now it's back to waiting.


Radio power output. Goes to blue connector, grey wire with red stripe.

Power side of radio fuse continuity test to main power lug.

 

I'm happy to report that, so far, there has been no power loss to the radio since the disassembly of the fuse box, even though there were no obvious defects, corrosion or other issues.
It was a comparatively simple job, everything came apart easily - this video was very helpful.

 

So it lasted for 2 months, then same power loss to radio that resolves by pulling and reinserting the radio fuse.
Next time I'm going to test the output wire in the blue connector. Just weird!

 

Update.
Went to test the power output wire on the blue connector so I lifted the fusebox, went in the truck and turned the key on. Yeah radio had power. So the grey wire with red stripe was able to wiggle. I took a small ziptie and pulled it tight against the fat wire next to it. Ghetto I know, but if the radio loses power again lifting the fusebox won't wiggle that terminal and I can pin test for power at the terminal. Process of elimination.

 

Ok,so new question... The next time the radio lost power I tested the grey/red stripe wire as it comes out of the blue connector and had voltage.
I've been looking for a good schematic and this is the closest that I found.
The odd part is that removing power by pulling the fuse appears to reset something downstream. It's the only explanation for how it's behaving.
So what is downstream?
I'm getting closer to just running a new power wire to the radio.... but really want to fix the issue.

Downloaded schematic

 

I gave up.....
wired head unit to the key-on power from the driver side cigarette lighter socket, and the constant power from the passenger side socket.
Radio and cameras work now.
The headlight left on chime tells me when the old circuit doesn't work (again), no chime = reset fuse.
So now we'll see if the head unit power draw was the cause. Not ready to buy a new fusebox just yet.