I also had what I think was galley water, come out every time I opened the main cap at the dump station. It was just enough to get my hands messy, and splatter my shoes (about 1/2 gallon). I have a 4 season model, with the valves enclosed in the belly. The galley dump line is lengthy, from the valve to the main sewer cap. It had a little rise in the middle, and didn't fully empty at the dump station. The next time I would dump it would be right at the cap.
I figured this out by removing the cap, after putting a bucket underneath, when I got home. Now the next time out, when at the dump station, nothing came out when I removed the cap. This tells me that my valves don't leak!
I think the OP's dealer may have been right, about some water staying in the lines, but moving toward the cap outlet, on the trip home.
This may not be the OP's only problem, but could have been most of the initial problem.
I did buy the twist on valve, just to not have to deal with the small mess when dumping.
Jerry
