I thought the galley is the same as the kitchen, but maybe Jerry427 means that the outside kitchen in the back is considered the galley and it is what shares a tank with the 1/2 bath?
Since the kitchen has its own tank, the gray water from the kitchen should not be coming out of the same pipe as the 1/2 bath gray water, even though for drainage purposes they might link up. It does sound like a valve is not closing. Of course we're not under warranty now. Any ideas for locating these valves or finding out where they are located? Of course the answer is probably that we have to take it to a dealer or Camping World and pay hourly while they search...