I had the same problem on my atwood HWH. I pulled the plug, removed the pressure release valve at the top, and drained the tank. I reinstalled the drain plug and added a 50/50 mix of water and vinegar to the tank. Left it for an hour, drained again, reinstalled the pressure valve and the drain plug. It was not difficult and fixed the problem. No odor a year later. I think I had a bunch of crud in the tank.
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