Had a quick look at the aircraft in the OP.
The 737NG* uses a vacuum toilet** system. All the paraphernalia (vacuum pumps, tanks, lines etc...) sit below the centreline naturally.
Same with the potable water and drainage system. Even that is more complicated than you might think: collectors, heated drain masts, heated drain lines blah, blah.
Looking at the P8, the lower fuselage has been comprehensively butchered (where a lot of the above is located and routed) to make room for the warry stuff. The E7 less so.
Hence, bye bye sinks and sh!tters.
* One of my engineer type-ratings.
** Note to illegals: no, flushing your passport down the bog doesn’t get rid of it. It doesn’t go very far. Thank god for rubber gloves.
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