Just to add to what Festus said with a funny story... my in laws were using our 5th wheel and we got a call that the sink was backed up in the kitchen. I went over to assist and noticed a plunger outside the door. When I asked what that was for (my trailer, I want to know if something is broken) mom in law says we tried to plunge the sink... didn't work. I informed them that all the water they washed down the sink had to go somewhere and that somewhere was a small tank under neath. That has to be emptied periodically. Anyway, emptied and the water drained. I have also found that the toilets will fill into the pipe and really cause you not to want to use them...