Funny, I registered here over a year ago if I recall correctly, and I get on and read a lot, yet I guess I have never actually posted. Funny that.
Anyhow...
A few thoughts...
First you said this is going to be an expedition rig, yes, in your first post?
That in mind my first thought was 'HOLY CRAP that is a HUGE bathroom!'.
Are you serious with that thing? I mean it takes up a full 1/5 of your space in there...
Never the less, you mentioned the composting head, and I thought you should check out the "Eco Roamer" build thread over at the Expedition Portal forum. Jay Shapiro is a friend, and they covered that head with exhaustive length in the thread. One thing that came up, and my be relevant to you, is that with those things you really have to let it compost, with nothing new added into it, for about three weeks before it can be legally dumped as dirt. If you are planning to live in the thing for weeks or even months/years then you may need two or three of them in your trailer just to be able to allow for proper composting.
Another thing I have seen often is to put the gray and black water tanks on the tow rig, this way you can drop the trailer and go dump the tanks without having to move everything. IF you did do that, you could allow for a slow drip to drain the composting toilets waste liquid onto your exhaust pipe allowing it to simple evaporate. This is something else that Jay did on his truck.
Anyhow, it looks like a great platform for a build! If you are looking to paint it, you can get some "bus kote" paint. This stuff goes on about 1cm thick, and works as an insulation as well, which is always nice to add a bit more insulation onto your expedition rig. I believe they only come in white, but that you can have them tint it a different color if you would like.
Anyhow, again, super cool idea, and I look forward to reading the conclusion to this thread!