[VAC] grey water puzzler

[Cathleen Day <cathleendaytx@xxxxxxxxx>]

I am so thrilled to be cleaning and learning the systems in my new 1977 Argosy Minuet, (20 ft.)  But I have a problem....  When the grey tank dump valve is closed and the tub stopper is in the shower, the kitchen & bathroom sink drain quickly into the grey tank.  If I take out the tub stopper in the shower, the bathroom sink & the kitchen sink back up and that water percolates up thru the shower drain.  When I open the grey tank dump valve, the shower, kitchen, &bathroom sinks drain quickly. I'm baffled!  Got any ideas??? THANKS!