Thanks, folks. I drained and refilled the tank, and the smell went down 90%. I suspect another drain/refill cycle will clear it up.
I saw no way to add CLR or vinegar to the tank without putting it in the whole system. There are clean-out nozzles for later heaters; perhaps shooting some vinegar solution up inside via the drain plug?
It is, by the way, an Atwood with an aluminum lined tank and no anode rod.
I'm in business again, and I now better understand one more of the many things that can go sideways in an RV.
Rich