If you're in a larger aircraft (typically with a single point I might add) then it's easy. You hook up the blue tube and the poop tube to their respective areas. Empty the poop through the poop tube and put in new blue juice. Easy and you don't get poop on you if you do it right.
The "tray" lavs though.... They are literally a tray that comes out from under the seat. The poop sloshes around and then you have to take it into a bathroom or sewer and dump it (btw, it's almost impossible to empty out of the weird square hole completely) and then put in whatever blue juice it calls for. I got one of those once. It had dihariaha and vomit mixed together. Worst day ever
That, uhhh, paints a pretty clear picture! (Or not so pretty!)
I like the idea of "the line guys do it"!