Is it a compromise?
How old is the boat? My 36 Classic, is screwed together. Remove everything from the wardrobes, and see if the back panels are screwed in place…
Remove and see what space you have behind…..
On our 2 cabin 36, in the wardrobe space that is forward of the heads, I took the back of the locker and found a space I recon I can put a 110lt tank, without loosing the locker space… its within easy reach of all the pipes in the heads locker as well as being under the side deck for a deck based pump out.
I am yet to template up the tank and have it made… But its on the project plan.