I had the same boat and my holding tank was on the starboard side of the boat, in the bilge area, between the fire wall and stern of the boat (just aft of the water pump). I ended up having my father in law weld a custom stainless steel tank as my plastic tank started to leak. You obviously want to place it as close to your head as possible to eliminate a long travel from flush to tank. Good luck!
Job done, as you said I installed it on the starboard side, behind fresh water tank.
I think it's balanced out okay as I have 4 batteries and the hot water tank on the portside.