DEMO CONTINUES...
Been traveling and only had a few hours this week. Here's what's new:
Opened up more of the wall from the living room to the bathroom. (Don't want to mess with the bathroom sink and wall behind it yet, as it's been winterized and I haven't drained the lines.) The fact that there is this much space between the bathroom floor and the underside of the rig (and the tanks) is probably going to be a great thing for doing work and maintenance:

Looking up close, I've wrapped orange tape around the toilet waste line. The smaller waste line on the left (and running diagonally to bottom) is for one fixture (!): the sink on the rooftop deck.

My Plan A for reconfiguring the bathroom is a layout like this. Ignore the box to the left of the shower, that was the cabinet in the living room (now removed):

...this would require the toilet to be moved from the back left corner of the bathroom to the back right (where the shower is now). The big question is whether there is room to run the waste line from the toilet back over to the black water tank. I'm skeptical, but still holding out hope. Here are two photos showing the room to work with:


Here's another interesting thing. There was a triangular chase built in the corner behind the toilet, which I opened up:

The plumbing runs up and into another chase, behind the bathroom wall (at the top). This area is directly over the living room ceiling, in the area that backs up to the bathroom (used to be cabinet). What you see here is the supply line to, and the waste line from, the sink on the rooftop. The the other side of this back wall is the edge of the rooftop deck:
