George,
We had similar symptoms when our rig was a few months old. Finally one day I parked next to the infrequently used dump at the Bend Service Center and waited the hour it took to drain the tank. I flushed it a bit with the built-in rinser and waited for that to drain. Then I used long sleeved rubber gloves and reached up beyond the black's valve with a coathanger I'd bent a little crook into. Poking around and pulling, it wasn't long before I disgorged a 3 inch diameter piece of black plastic.
Turns out this is very common. When they install the waste tanks and cut out the hole for the valve fittings, the cut-out often flops inside the tank and is not removed because the guys don't have time to dig it out. It may not be an immediate problem for the new owner, but eventually that little disk will work its way back to in front of the outlet, with each flush of the tank. Then it flips up and can jam in the opening rather than float cleanly out.
Try the coathanger, George. If your last tank emptying procedure was slow-going, and you have the same problem I describe, then that little disk is right there at the valve cutout and can be easily fenagled out. I fully thought I was going to find some kid had stuck a towel down the john while the coach was showroomed, so was surprised when my coathanger drug out that disk within 20 seconds.
