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Hi my name is Robert....
First thing to do is make sure they have the drain for the washer going to the gray water tank, most are set up like that. The tanks should be separate as well, the grey and black water should not mix at all. When the black tank gets full you would see it at the toilet. The only other way I can see that happening is if you have both discharge valves opened then it might want to fill each tank but the valves should be shut anyway until the tanks are at least 3/4 full. It sounds to me it could be plumbed to the black tank. Might want to have a RV shop take a quick look at this one...... Robert
Also what I might do to help confirm where the water is going is hook up to a sewage connector and then open the grey water valve and see if water comes out, if not open the black tank valve. IF it starts coming out the black tank right away then it has to be plumbed into that tank.
Let me know if you need more help on this, there is really no way the black and grey water should mix, if it is plumbed to your black tank you will need to have the black tank valve open so the tank can drain, I am not sure if you tried running it with the valve open or not..... Also make sure the roof vents are clear and clean, if the black OR grey tank vent was obstructed it may not allow for proper drainage. Robert
Was just checking in to see if my answer helped?? IF you have more questions or concerns please feel free to contact me, follow ups are included if needed.... Robert