2018 RT27-OB water heater problem

[John Cortesy]

My first request for help, so I hope I am doing it right. HIN number FMLT2744L718. The ISO temp heater, which runs off 120 electric only, will only get the water luke warm. The mixing valve has been set from full on to full off with a one hour wait each time to see if there was a difference in the setting. There was not. The breaker is on with a green light showing power to the circuit. The cover was removed and the reset button was pressed. One hour later there was still no change in the temp.

The possibilities appear to be a stuck mixing valve, a bad heating element?,a bad service thermostat, or a bad safety thermostat. Is there a way to troubleshoot those parts with a multi-meter so I don't drag this repair out for weeks?

Thanks,
John Cortesy