I don't have a plumbing diagram or anything like that. However, attached is a snipet from a doc I have that might provide a lead to get what you are looking for. It is a parts doc that indicates the part numbers for the 'optional head'.
28-123 Head - Portable
28-091 Head - Portable w/Deck Pump-out
28-151 Head - Portable w/In-line Macerator
I doubt those numbers work from 18 years ago, but may provide a starting point for finding more info on the plumbing and electrical.
I was interested in doing something like you are contemplating in our 2004 Gulfstream; happy wife, happy life kinda think when my wife is aboard. It turns out to be fairly involved to do. The water supply is fairly easy, but if you want a holding tank (needed at many Southern California harbors) it gets challenging. Instead I decided to stay with the porta-potty and change it every couple of years as the seals dry out and smell funky. That's my $0.02.
