If you have it, you can't miss it. Mine (28BHBE) is on the rear wall next to the city water inlet. It has a big silver sticker above it that warns you to leave the valves open while operating the flush.
I actually flushed what I thought
was a "clean" black tank into my side yard after one of our first trips (gasp! Yeah, yeah, I know...). Needless to say, it was NOT clean, so yes, the built-in flush works well. And no, I will NOT do that again...
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!