go to the hardware store and get adapters to convert the pressure relief valve "hole" to garden hose thread. attach garden hose to the new fitting, and then pull your anode rod from the bottom. let water flow through that tank (gas shut off!) for hours and then give it a sniff.
alternately, mix up five gallons of baking soda and cold water. use a lot of soda. pour it into the relief valve hole, let it sit for a few hours, then drain it from the anode rod hole. repeat two-three times. baking soda will attack the hydrocarbon esters (oils) that are giving you the smell.