(Can the grey water tank be full? The shower may have one grey water tank, and the sinks have a different grey water tank.)
This is like, "My car won't start." And then you find out it's out of gas.
The handles for the tank drains are all in one spot. I have two for black: one the full bath and one for the half bath. There's only one handle for the grey. I checked outside that compartment and there isn't anything else. The water in the kitchen eventually goes down, That's why I believe its a clog. The full bathroom and W/D at the other end of the trailer drain fine. I think they sell a drain cleaner for plastic pipes, so I'll head to Lowe's and see if I can find it.