Sulfates or hydrogen sulfide gas in the water giving a distinct "rotten egg" smell.
Some places have a high sulfur content in their water. Left sitting in an RV for awhile can bring out the worst. Simply drain the water heater and flush out the system with fresh water. It should all go away. If this is happening frequently and your water heater has a magnesium anode, change to an aluminium one. There are places I have camped that have a high sulfur content and I drain and flush the system when I return home.