Slowly and with plenty of witnesses invent the toilet. But like out of wood pieces like a barrel or ship. Rain barrel on the roof for water. Start suggesting more contained sewage.
Should be just enough to not get dead for heresy or something but live comfortably and help a shitty situation.
Human urine was collected and used for many things (mostly the ammonia). Human feces was used as fertilizer in a lot of the world until very recent times and collected in certain areas.