Alas, try as I might, I can find nothing wrong with the panel ... plus, with it installed indoors, recessed into the wall, AND it being a trailer (nearly impossible to patch) ... I'm not about to change it. Zinsco lives on ...
Today was productive, in that I now have a very good idea as to what is wrong with the trailer. This, my friends, is why you need to have a megger!
Anyway, I need only replace two sections of wire. Naturally, the least accessible one is guarded by a 35 pound black widow spider ... at least, that's how it looked while I was under the trailer. I just PRAY the brick I hit it with didn't just make it mad!
So, it looks like I'll be doing a fair amount of pigtailing. Apart from the GFCI's, I'll need to do this at the light fixtures.
The King "AlumiConn" connectors look to cost in the neighborhood of $3 each. That's ten bucks' of connectors in a box. I suspect I'll be 'daisy chaining' the $2.60 Cu/Alr devices, rather than my usual pigtail method.
T&B, who had assured me at "counter day" that I would have no trouble getting their rated "Marrette" connectors (about 40 cents each) is proving elusive, now that I actually want some.