Black Water filling up with clean water after I have drained it

I have a 2019 Grand Design Reflection 5th wheel a new problem encounter after I had all of the tanks back flush. One of my Black Water Tanks after dumping will show 3/4 in about 3 to 4 hours, I dump it again and the water comes out clear. I have checked the toilet associated with tank and does not appear to leak and all connections for the toilet look good. In the cupboard that has the connections and controls I've looked on the back side of the panel where all of the connections to the various water lines and its dry? Any thoughts are welcome
John

 

i dont know how you have things set up with the hoses, but is there any chance your back flush hose is still hooked up and slowly filling the black tank? I know, silly question, but thought i'd ask.

You imply you have two black water tanks. do you mean you have a gray tank and a black tank, or are there really two black tanks?

 

How many ways to get clean water into that thank? Is the black tank maybe hooked to a shower?

 

I've seen bathroom sinks plumbed into the black tank, but never the shower. I don't think the RVIA code allows it since the shower is the lowest point in the bathroom if the tank over fills.