We have a holding tank in both heads, around 75 litres each. They were fitted by the first owner when the boat was fairly new. Go to photos and search Santorin holding tanks and you will see the installation there.
I am not familiar with the interior of the Maramu, but the Santorin looks the same as the Super Maramu. We've lost the clothes hanging drying area as the tanks fit that space and the pipework does fill most of the alert side storage areas. The tanks are visible but not unsightly. The pros are that all maintenance is easy, access to valves, macerator pump tank interior access ( not pleasant but useful!) and of course the tank level reader.
I haven't spotted it yet, but somewhere there is a schematic of the pipework and tank dimensions.
In my previous boat I had stainless steel tanks. Avoid s/s for holding tanks!!
Ocean Hobo SN96 North Wales (UK!)