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Hi my name is Robert.....
You said the shower is backing up with black water? That should never happen as the grey water and black water should be separate. Make sure the valves for the holding tanks are not open or the black water might be able to get into the grey water. IF you are hooked up to city water make sure you did not hook up to the tank flush if you have one, that might cause the tank to back up. IS your black water tank reading full right now? Normally the discharge valves for the black and grey are right next to each other where the discharge pip is located, let me know if the black tank is reading full.... Robert
Also sometimes you will find another valve near the rear tires sometimes the slideroom needs to be out to see the valve, I would look above the tires real quick and make sure you do not have another valve... Robert