Hello from Colorado jdbuilder: I think most of new Airstream trailers have the bathroom in the center of the trailer. Our 86 Limited does. Rear bedrooms make more sense, but more importantly getting the water weight toward the center of the trailer makes sense. Further, Airstream has been using below the frame rail sewer discharge ports for a long time.
That said, my 75 Overlander is a rear bath model, with the waste water tanks located in the rear, the the discharge port through the frame rail just before the rear bumper. Not the greatest idea Airstream ever had.
I used to own a 66 Trade Wind 24' tandem axle. It is a rear bath. I reconfigured the waste water system to include a nice sized gray water tank and new black water tank. I routed the dump valve manifold below the frame rail. Yep, it took away some ground clearance. I find it effective in scraping dead raccoons of the highway.
I recommend building a street side below the frame rail sewer discharge. Since you plan a center bath, make the drain port close to the rear wheel like our 86 has.
WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !