Ill go out on a limb here at the risk of sounding a like a jerk. But folks like to help but an accurate description is needed. (A picture of the cabinet door open showing the valve would be more help)
I also have never seen a dump valve that turned..your description of 'turning' is confusing, though in earlier post you also said said 'push or pull' so Im thinking its just description. Not holding your feet to the fire over terminology but just trying to 'see' the actual issue.
Assuming you have standard dump valve the handle should be pulled out, allowing tank to fully drain. If the tank IS empty and the water backs up into shower then you may indeed have some blockage. Some showers the drain screen is removable, where you could fish down the drain a bit.
when he turned the handle outside the camper the water came out so that's where im stumped water is going into the grey tank but some up through the shower drain. unless when you pull the handle the line blocker thingy ( I don't know what it called LOL) doesn't close or open all the way when you push it in or pull out...