Anybody see anything wrong here for my water hookups?

I don't see any problems. Actually, it's pretty creative in my opinion. And I see nothing wrong with hook-up for the black tank flusher. The anti-syphen check valve will prevent anything from coming back feeding.

And ... you still have one "port" open to attach a garden hose to with a sprayer to spray down the muddy kids or the muddy dog! (or to rinse off all those fish you might catch and bring back to clean!