Black and Gray Tank issue.

I have a device that allows a hose to hook to the 3" drain line. I can control which tank the water goes into by the individual tank valves. Water goes in, but I have yet to figure out where it is actually going. The sensor lights are showing 3/4 for black, and 3/4 or hovering at 4/4 for the grey. I noticed after all the fooling around, when I flush the toilet, it sounds like the tank is empty. Now for the last couple flushes, it sounds like some water is accumulating. I am going to add about 5 gallons of water into the toilet and see if I get a noticeable output. My next step is going to be to open the grey line at the valve and attempt to replace the valve. It is getting difficult to operate. If it leaks good after opening, I will try to drain the tank then replace the valve.