It's a suburban. I don't follow having power to the element and relays and breakers. Seems it should work with power
Thanks for letting us know the manufacturer. Suburban and Atwood have very different control systems. For electric, the system is much simpler.
So....you've already replaced the element twice. And you're measuring 120V AC between the element's terminals and presumably a nearby ground. That argues the thermostat and emergency cutoff are ok. You said it works ok on propane, so that would argue that there's water in the tank and you're not just burning out the new elements.
That leaves the neutral connection as the main suspect. If that line is open somewhere, you can have all the power in the world getting to the element with no results.
If you disconnect power (for safety) and disconnect the wires from the element, you can take a continuity reading from the neutral wire that goes to the element, to a neutral pin on a nearby outlet in the UDC. If it's open, the next place to check is probably the j-box on the front corner of the water heater. If you're not sure what's what, take a look at page 9 of our Water Heater Troubleshooting Guide, in
this folder.