Originally Posted by Gary - K7GLD
Sounds like the leaking valve ran the tank empty - and the still running burner cooked the tank...
Not possible. The TPR valve is at the TOP of the tank, for it to run the tank empty the RV woudl have to be parked...er.. "Wheels UP" (as in Shiny side down or if you like UPSIDE down (on it's roof)).
However some of the other ideas are good.
If the water heater was not used for an extended period of time it is likely a biological has taken hold inside the tank (Bacteria) and is growing out of control. The heater needs to be flushed and sanatized, You do this the same as the rest of the system only open the TPR valve manually so as to get as much water in the tank as possible.
When you are done again drain the water heater. (TPR VALVE STILL OPEN) then close the TPR and then replace the drain plug.
If the water heater is a surburban you may need a new anode rod, Black water is a sign of the anode rode haveing gone into solution (Among other things) the black is actually zinc oxide