Norcold Fridge Issue - Help!

Okay, I turned off the fridge and let it sit overnight. Tested the new, recently replaced AC Heater and it tests at 44 Ohms. I'm thinking about turning the fridge on, waiting for the element to fail, as it seems to run on AC for 30 minutes or so before it fails. and then ohm it out again once it fails. Perhaps it's failing after the element begins to heat?

ohm the element out. If the spec is ~2.8A you should read ~43ohms. (120V/2.8A = ~43ohms).

If the element is ok, there is extra resistance from somewhere else including wires, connections or the relay.

The relay in our 621 failed but more drastically. I replaced it for less than $6 (http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29727778/gotomsg/29741786.cfm#29741786)

I would check for output from the circuit board. The circuit board is looking for resistance of the element. It is possible that the relay is bad on the board. you could wire up a standard 120 volt lamp socket with lamp and connect to output of the circuit board to simulate.the element. If thelamp does not light up, the circuit could be at fault. 60-100 watt incandescent lamp should be sufficient.
Measure your replacement element for resistance across the leads. No resistance means it is open and bad.

I'm hoping it's a bad element; the last one worked for the five years I've owned the motorhome without any issues.

Sounds like the new element burned up. Either you got a bad one, or something is wrong that is burning up elements.

Yes, I am aware of that, which is why I replaced the AC Heater with a new part.

C6 code means AC heater open...

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