Do you have a valve tap that you need to turn to release the Grey water or do you flick a switch so that an electo/mechanical valve opens to release the grey water, if its an electro mechanical valve I would check to make sure you have 12v power to the valve, ie could be a blown fuse.
If the Grey water release valve is mechanical would look at the rubber boot type of thingo with a handle that you could but over the sink ( full of water) and push down to put the system under pressure. Would make sure that the shower water outlet is blocked first.
If other comments from the forum cannot help would put your request onto the OZeRV Forum where a number of CMCA members are after the CMCA Forum closed down, many would own Jayco Conquests.
Thanks Peter, we managed to get it all fixed, it is a manual empty - took the outlet pipe off - it was blocked with something that had gone down the sink - ooopppps! who does the washing up
All fixed now - will see if I can find a strainer thingy for the sink - but those small plug holes are a bit painful to get bits for I find.
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jules "Love is good for the human being!!" (Ben, aged 10)