The Threads are usually 1/2 inch IPT. Atwood normally uses a plastic plug I suggest a six point socket (not a 12) on this plug. Suburan normally uses a metal plug with an anode rod. this one can take any kind of wrench.
page 2: Keep a short length (like 6 to 10 inches) of plastic irrigation pipe, also 1/2 inch thread With water heater cold and pump /city water off. pull plug, thread in pipe open TPR valve to drain (Stand back) pipe takes water out past side of RV. Need not be all that tight. Finger tight will do.