[JRTJH]

When you pull the valve handle on the galley gray tank can you reach inside and feel (or visualize) the valve blade? Is the valve actually opening or is the rod/handle moving and the valve remaining closed? It sounds like the galley holding tank is full and not empting. It does not sound like there is a clogged drain line from the sink to the tank. From the photos I've seen of your trailer plumbing system, the entire valve is exposed below the trailer belly covering. You should be able to reach into the drain fitting and feel the valve blade. I'd suspect the handle has either unscrewed from the blade or the attachment rod fitting is broken.