Many thanks once more for everyone's support. It really helped me to (painstakingly) locate the obscure source of the problem. For those in a hurry, spoiler alert: waste compartment light switch crimp fail.
The search: I ran a live bypass to Red, from the DC board to underside the bed. Once connected to the Red that feeds the slide lights, they all lit up,as did the USB charger on the same circuit. The Grey feeds the 'Cigarette lighter' charging point under dinette and the CO/NG monitor (Orange).
So the first question: where is the break in the Red? I inspected the undercarriage for any damage to exposed cable ducts ... nothing.
Last port of call was to check the two rear left side compartments. While these are not part of the 'slide', I suspected they might be fed by Red.
On a hunch (working with fingers, mirror and flashlight) I found the live feed to the light switch crimp fitting in the waste compartment had detached. My "aha moment": the Red under-bed feed doesn't come directly from the DC board, but is looped via the waste compartment light switch, before it heads to the under bed splices, after which it feeds the slide side soffit lights etc. It makes no sense to me that it was done that way, but FR must have had their reasons.
It was - and is - a spectacular weak point: the Red 12V feed to the female crimp on the live side of the switch was poorly done and mangled (just about twisted off). The live wire eventually detached from the crimp, opening the whole 'slide circuit'.
To top it off, three spliced earth wires on the same loop in the waste compartment were only twisted together, having never had the benefit of a crimp fitting. In time, the spice would also have disconnected.
The 12V Red and 6 tank sensor leads (3 for Black tank and 3 for Grey tank) had also been tied all together and pushed up into the space between the black tank and the sidewall. Fifteen thousand miles later, the whole nest dropped down into the compartment in front of the waste valves, at our last stop before winterizing.
I suspect, but can't prove, that the wires had originally been hung off the light switch crimp, just above the switch and out of sight. If so, this would’ve contributed to the crimp deformation and Red wire pulling out of its crimp. I reconnected all, with a new crimp, and suspended the wires in a cradle from one of the threaded tank support bolts, out of the way of the waste valves.
Other stuff learned:
o "Slide" on the DC board legend means: "all 12V load points on the slide, including LH compartment lights".
o The under-bed Red line feeds three downstream circuits:
o Red (lighting circuit including USB soffit charging outlet)
o Grey 12V ('cigarette lighter') charging point below the dinette
o Orange - CO/NG monitor.
o The under-bed Green line remains a mystery - it's still dead, and I couldn’t find out what it’s supposed to feed (the furnace is fed by Green from the 12V board, and it works, so it’s not
that green wire). Maybe someone out there knows?
OK. Hope that helps the next person!
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