Thanks for posting about this subject.
I am current looking into how to rework mine flush system as well. on my 2023 382RK the flush line goes to what appears to be an anti-syphon or backflow preventer up under the half bath sink. From there it goes off to the black tanks, and I'm not sure if a can get to the "tee".
My main concern is the flow is very slight at best. So i never feel I'm getting a good flush. Also when a shut the flush off, there is always a lot of back pressure, which tells me the system is not very free flowing from the hose connection to the tanks