While working in Iran I dated a FA from Lufthansa. She told me the following horror story. They were carrying Muslims to and from the Hadj and most of them had never seen a western toilet let alone the types of toilets found on commercial airliners. When they had fully soiled the toilets they picked a spot at the rear of the aircraft to relieve themselves (1 & 2). The smell became so overpowering that the cabin crew went on the flight deck carrying oxygen packs in order to avoid the stench.
When the aircraft returned to Frankfurt they not only steam cleaned the toilets they gutted the airframe down to an aluminum tube. She said it cost more to return the aircraft to service than they made in carrying the passengers.
On a more personal note I flew TWA from Malpensa to New York and the aircraft arrived late. The ground crew did not have enough time to clean the cabin so they took all of the blankets and pillows that had been used by the arriving passengers and piled them up at the rear of the cabin. It didn’t take much time and the rear of the cabins started to smell like the inside of a gym bag.
