I had similar troubles on my 216.
Had me stumped for along time. In my case, it was that one of the springs which pushes the brushes onto the armature, had lost most of its tension. That brush wasn't being pushed to make contact hard enough. The truck would start when cold, but not hot-the weakened starter couldn't spin the engine against the increased compression.
A new spring plate thing solved my troubles, after much, much head scratching and bewilderment!
My starter had been rebuilt by a local shop, and they had missed the trouble!
It also wouldn't hurt to stick the hygrometer in the battery, and see how charged it is, although with the double battery, and 12v tests you did, I can't see that being the trouble. I've just learned to sometimes start from the beginning, after several humbling experiences like this!
The great news is, that you've been systematic and thorough so far, big cables and what-not. You'll get it solved.
