We've got really similar trailers, Jeff, so maybe look at my thread linked below to see what I did with my gray tank. Haven't redone the black as yet, but it'll still be the above floor type in the head.
I'm planning two dump valves on mine. I'd love to do something like what I think you're describing, but I'm hesitant to get away from gravity and a straight down exit for the black tank. I want the black plumbing to be as simple as possible.
That's what I'm thinking, anyway.