BTDT with the aft cabin head line clogged. Years? Its probably rock solid all the way to the tank. Nothing you put in there will dissolve the piss-crete over that long distance. I had to pull and cut 1 ft at a time for 20+ ft to get it all out. Not fun but not impossible.
One more thing. Use premium odor barrier hose like Raritan Saniflex. That cheaper white 148 hose will permeate in less than two years meaning you WILL replace it again!!!
1984 Monk 36 Hull #46
Currently in Cape May, NJ