Don't know when you got your 3625 RE but if you winterized it and did not blow out or push the "pink stuff" into the black tank backflow valve or preventer and line it could have frozen and split the valve or the line. If you were snowbirding and discovered it down there somewhere, it could just be a backflow valve that has gone bad already. This is a common happening for those valve. If you are still under warranty, I'd take it into the dealer and let them fix it! If you want to do it yourself do a search on the Search Engine above on the subject of black tank backflow valve and you'll see many a post on this problem and a couple of fixes for it also-- replace or remove
. It's that simple BUT getting to it is not the easiest procedure. It's easier if it is under the sink, even though it is located up high under the sink than it is when it's located in the wall between the shower and the cabinet. Good luck and update us on what you find out!