[a big scary monster]

Hi Robert I sent you a couple of pm's since I came back of hols.
My dads motor home is wired like that alternator feed cable links to the battery feed to solenoid post, and randomly hot or cold it will sometimes just click like a duff solenoid, in the history of vehicle there are numerous receipts for starter checks new starters etc very unrelated to yours but I had wondered if the alternator is jiggered and putting out high/ irattic power if it was frying something, although surely the solenoid post is entirely isolated when the solenoid is idle, my next port of call is to run a new trigger feed to solenoid direct from a fused battery connection with a switch in the cab to rule out a Dickie ignition switch or intermittent break in the trigger wire,( his problem usually arises when he is in a queue to get off a ferry or at a busy petrol station never while calmly parked on a campsite)
Relevance to your issue and a long shot, could your alternator be doing odd things whilst hot? Stealing power or supplying high low power, i have had motors that didn't like to start unless the starter spins at a very specific speed, although usually deisels Ed