Keystone has "plumbed the tanks" in a variety of ways. There doesn't seem to be any specific "always standard" way that it's done and it can vary from trailer to trailer as well as from brand to brand.
On some, the kitchen sink/second bath sink share a gray tank, on some the bathroom sink dumps into the second black tank, on some the washer/dryer dumps into a tank (black or gray) and on others, it doesn't dump into a tank, but dumps on the ground or to the "sewer line coupling"...
As for the black tank flush, some are connected to both tanks and flush them at the same time, some are connected only to the "main black tank". Which is the "main black tank" ? Good question, only the guys on the assembly line really know for sure which it was on the day your trailer was built......
So, an afternoon of dumping buckets of water, opening dump valves, spraying tanks with the black tank flush system is the only "real way to know for sure" how yours was plumbed. That is, unless <tongue in cheek> you can find a daily schedule from Keystone that would tell you what's different in trailers built on Monday vs Wednesday.....