Plot Twist -
So.........dear hubby decided to help by draining the spa and forget to unplug it (110V). I came outside a bit later - with most of the water drained - and the unit was clicking to beat the band. I immediately unplugged it. The spa was refilled, balanced, and turned back on. It heated up to near 100 and then the temps outside got so cold we wouldn't set it in. Didn't pay attention to the heat until we tried it out a few weeks later. The heat is at 95 but won't go higher. I really think the tub is just maintaining the heat it had. I have done all the tests above.
Results - zero voltage across the heater relay board BUT the board has NO indications anywhere of any damage. Hmmmmmm........what now? The heater tested out at about 11 ohms. Both lights on the main board are one as indicatied. No GFCI issues. Could it be something else?