Question Black Tank srs365 RB

[gordo2050]

Geez, y'all are going to kick me off this site for all the questions.
So I bought this used rig, and even though I have been RVing forever, this beast keeps me guessing.
Prior to dumping my tanks, I used the "black flush" to fill the black tank to make sure I got it really clean. I stood inside, looking down the front toilet, while son stood at the ready at the hose. When I saw water, we shut water off, checked gauges to confirm full, and off to the dump station.
When I pulled the black valve, it took maybe 10 seconds to empty the tank. Considering that according to the owners manual, I have about 45 gallons of black capacity, I think it should take longer. The gauge shows empty, but I still find it hard to believe it empties that fast.
Is it possible I have another drain valve somewhere? This is a bath and a half, and based on indicator lights, I sure seem to fill the tank fast.
I've looked everywhere, I think.
Any advice would be GREATLY appreciated..

 

[Kevin D Pem]

Guess you could measure the deposit, could have, well! You know dried up in there.