We just finished building a tiny house on wheels. The bathroom really gave us fits in terms of design, especially since our project chief (teenager) demanded that it be off-grid, so gravity water pressure only. We finally figured it out, and it works pretty good. In case it saves someone else a bunch of headaches, here are some pics, and we would be happy to answer any questions.
We wanted a urine-diverting compost toilet (Nature's Head, great product by the way), but didn't want a special drain from the removable urine tank, since this is not a live-in deal but more of a "spare room" for teenagers and guests. And, males don't want to sit to pee, as a general rule, so how to fit a urinal in a 4x7' space with a toilet, sink, shower, and interior propane water heater? The photos show our solution, and it's been working great for six months.
The shower is built on a 24x27" pan with galvanized sidewalls, and is based on a Mr. Heater BOSS XCW 20 propane camp shower, which pulls water out of a 15-gallon drum. The drum is supplied by an overhead RV tank through gravity pressure, with a manual on/off fill valve. You can see and hear the water filling the drum, so you know when to turn it off during a refill. We get 3-4 comfortably long showers out of the drum.
I love love love the urinal under the sink that gets rinsed with hand washing! I built a dry toilet with a Kildwick separator in it, but would like to add a urinal so the men don't have to sit. The under sink idea is genius double-duty engineering that will keep a smell from forming too.