I measured going in, three gallons at a time, doing a visual inspection of the water level after each bucket full. The specs say 40, but I can only put 20 in it before it will overflow into the trailer.
And, as gross as it sounds, I measured the volume of what came out. It had been flushed a bunch of times, so it wasn't too yucky.
The reason I'm doing all this is because on our first campout, the black tank filled up after only day of use and I had to visit the dump site. I took it back to the dealer and they said it seemed to have had some old stuff in it, but was now flushed out, however, it appears to only hold 1/2 of what the specs say.
Being new to this, I'm not sure what to do next.