It sounds like you are saying the leak is at the fitting where the relief valve is screwed into the tank. I also gather that this is on the side, not the top.
It is possible that it is simply a fitting leak. To repair this would involve unscrewing the relief valve. ALthough it may not be defective, you would as a matter of routine use a new one. The consensus sealing method would be to put 2 wraps of teflon tape on the threads, and coat this with pipe joint paste. You will need a certain "feel" screwing it in so that you get to the point of just about tight, then tighten further until the outlet points in the right direction. You will probably need at least an 18" pipe wrench to get the old one out.
It is also possible that the leak is a rust-through leak at this fitting, or even at some other point near the top, and this is the point where the water finds a way out. You may have done this, but if not give the whole WH a going over with a flashlight, to see if you can narrow it down.