I have a problem with my kitchen sink. I over filled the grey tank and the water backed up into the kitchen sink. I drained the tank and now the sink will not empty. I'm reading that there is probable a 'in line check valve'. But I can not seem to located it. Under the coach is a huge skid plate, and the pipe from the sink goes below the floor. The storage compartment is accessable, and I see the pipe, but no where can I see where this valve could be easily changed out. I've read that it should be easy to access the valve. I can't find any kind of diagram telling me where the valve is or how to access it. Anyone have any ideas?? Thanks