In most RVs they are called the grey and black water waste tanks since they are not for drinking water. Most RVs come with a fresh water tank for drinking, washing and flushing the toilet. Then they have waste tanks to temporarily store the waste water until you can find a dump station (sani-dump in Canada) to get rid of it. Water from the toilet goes to the "black water" waste tank and water from the sink and shower drains goes to the "grey water" waste tank. Some RVs have one waste tank it all goes into and some have two grey water tanks so the kitchen has its own tank.