I had the same problem a year ago. We used water at a campground that has well water and then put the coach into storage for about a month. When we used the hot water after storage, we got the same smell. It is a result of a reaction between the sulphur content in the water sitting in contact with the Aluminum Anode Rod.
The only way to clear it up is to remove the Anode Rod, drain and flush the heater, then using the water pump, pump a couple gallons of Bleach into the heater in the same way you would pump Anti Freeze to Winterize. Close the valve out of the heater, Leave the over pressure valve open until water flows indicating the heater is full of the water/bleach solution. Let it stand in the heater for at least 4-5 hours. Drain and flush again and you should be back in business. It is a nasty smell for sure.
Dennis & Carol
2003, 4006, 41' DutchStar,
Spartan, ISC 8.3L W/BanksPower - 2013 Honda CRV, BlueOx Baseplate, Aventa Bar & Patriot Brake. And the 04 Bird, Sunshine Car.