We are the new owner of a 2011 RK3000.When trying to empty the waste and gray tanks yesterday I noticed there is a third dump at the rear of the trailer but nothing will come out.In the docking station there is only 2 valves.The monitor inside says 3 tanks and is telling us the 1 tank is still 1/3 full.Just do not know how to dump it?? It doesn't happen to be a vent would it and if so why is the 1 tank saying 1/3? Isn't that stupid I can not figure it out.Some help would be very appreciated!TIA