My shower takes forever to drain . The pipes could probably do with a good clean out, however I was looking under the van for another reason an noticed that the drainage pipe runs up hill from the bottom of the shower drain??! Also, on a few occasions I've had unexplained water appear in the shower tray. I haven't yet looked at the pipe work in detail but on other forums I see that sink water can end up coming up into the shower. I am minded to modify this odd setup and create a separate drain pipe for the shower. Has anyone done this before?