You might look for plans for building a truck camper. I believe most of those use a "wet bath" that contains both the shower, toilet and sink combinations. The plans should include a materials list you could look over.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine