I was convinced the Element in our Suburban SW6DE Water Heater. I must have mis-tested it. What it turned out to be, was the NEUTRAL wire, from the Water Heater back to the Breaker Panel ("Load Center") was open at a connection.
You can test for this at the Element. If the Element is GOOD, the Black Wire side should show HOT (120 VAC), but the White side should NOT. If (again with Good Element), Both sides of the Element Connections show HOT, that means the Neutral is OPEN, and it won't heat in Electric Mode.