The symptom was as soon as I turned on the charging circuit (truck battery to Redarc/LFP battery) I would get a voltage drop at the Redarc input,,,
At idle, the voltage drop was quick, 30 seconds or so. My wife raised the engine RPM to 2k and I would see the voltage rise at the Redarc and things would remain relatively stable for a time, but overall still drop slowly.
So then I purchased a charger with a built-in alternator test function (needed the charger anyway) and hooked it up. With no load and engine idling, it tested ok. then I added all lights, both radios and a/c (left the second battery disconnected) and tested again, and this time the alternator failed. It simply can't keep up with the demand, even without the second battery system. A rebuild will probably help, but I figure with the power draw and the potential to add more accessories, an upgrade seems like a good idea.