[jimcol]

Mikey, It may not be the check valve. In fact if you hadn't use the black tank flush any time around when you had the problem it probably isn't. The drain assembly and piping out of the shower have been known to leak. You could have a loose connection on the water lines feeding the shower. You could have a wall panel leaking where it meets the shower base. Turn the water on in the shower, go underneath and see what you can see. Some people experienced leaks in the base of shower but only when the shower was occupied. The weight of a person flexing the base or drain caused the leak. Hope this gives you some ideas of what to look at. Let us know what you find. Jim