At this point I'd suggest a call to the factory. See if someone there can tell you exactly how your plumbing is supposed to be put together.
I don't what your coach's floor plan is like, but if the kitchen sink and shower are almost back-to-back, I suppose it is possible that the kitchen sink drains into the shower drain and then into the grey tank. If there is a plug below that connection you could get soapy water from the kitchen sink coming up in the shower. Not too likely, I admit, but might be remotely possible.
Mercury Mountaineer towing mpg 181(both sold)
1993 Foretravel U300 40'