For the plumbing, and really for all your system decisions, start with how are you going to use your AS: Full time, part time, occasional weekends, boondocking, parks with full hookups, spare bedroom, etc..
Because we wanted the ability to boondock, we added a large black water, 2 grey water and 2 fresh water to our 58 Overlander. That would be a bit much for your FC. You also have options to carry spare water in your TV and a portable tank for grey/black water.
However you approach the project, try to plan out all the refitting before you start. Just like a house all the systems will overlap and here space is precious.
The forum members are glad to help. Giving us your initial plan (even if you don't end up using it) it will help us give you better suggestions.
Good luck, be patient (we're on year 4), triple your budget, have fun!