If the pressure relief valve has any debris or gunk under the seal, then it may cause an occasional leak. With the heater off, and the water cooled, pop the pressure relief valve,let it flow a bit to wash out any unwanted gunk. This may allow it to reseat itself. If it continues to leak, it's either defective (easy, low-cost fix) or your temp is set to a higher setting than the relief, which nad be dangerous, because your water will be hot enough to scald.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter