Yankeflyer, I plan on doing something alot like your gray tank on my gray tank.
But for now I'm just tying the shower into the black tank. It is alot quicker right now to do that. I am making provisions though to run the drain to a gray tank later. Some of the plumbing in progress. Not done yet. I have another ball valve that I'm going to add just next to the T, that is where I will run the line later over the frame under the bus floor over to the other side for a gray tank. There just wasn't enough room to keep both tanks on the same side. Fuel tank is in my way. I'm ok with it though. The black tank is 55 gallons, I figure I will probably want to add some shower water in there just to keep enough liquid sloshing back and forth anyway. I figure I'll have options this way.
I also got the vent ran through the roof as well. I have been putting that off for a long time. I really hate cutting holes in the roof.