Not sure of your model, but on my 2013 340 RK the sink tank dump valve handle was hidden UNDER the alum. skin of the trailer. Same thing happened to me. Put some water in tank and tried to figure where or how to drain it. Opened cap and whoosh. I had to remove the alum skin from the left rear of the trailer, get the dump handle out from under it and replace the alum. Then had to fabricate a support for the pull rod. I guess the plumbing goes in first then to the skin part of the line. Make you wonder what kind of folks are putting these things together.