The Report:
First, checked battery voltages with switch off....starting was 12.73, deep cell was 12.25
Motor cranked right up this AM with switch on ALL.
Gauge read 13 at idle, and seemed steady.
Idled smoothly out, cut her off and fished a bit. Pretty windy/rough and not much was biting.
Pulled anchor and proceeded with phase two testing......throttle.
You can't imagine the smile on my face as I watched the gauge move up to 14 and sit there under good throttle!
No spikes to 16, no alarm, no need to turn lights on, just a nice running boat!
Moral of the story.....
if you have an old Merc with a 9 amp stator and therefore, no regulator, you better have a good marine deep cell in the circuit!
This does change my two battery approach (from using starting for start/run, deep cell for house/backup to just use ALL) but I can adjust.
Hope all this helps someone in the future with similar issue. Sure seems to have solved things for me (at least for now.....)
Thanks to all who shared their thoughts!