The three main recreational vehicle (RV) water tanks are designed to contain fresh water and two types of waste water. A fourth type of water tank found in many recreational vehicles is used to heat up water for taking showers, cleaning dishes, and similar functions. The biggest RV water tanks are usually for fresh water, and they tend to be a little bit smaller in volume than the combined size of both waste water tanks. Most RV tanks for waste water are divided up so that the sinks and shower drain into one and the toilet empties into the other. RV water tanks are typically made of various plastic substances, though metal and other materials may be used as well.
Most motorhomes, trailers, and campers make use of three separate RV water tanks. The largest of these tanks tends to contain the fresh water. This is usually potable water, though it does not necessarily have to be. Fresh water RV tanks can be made of metal, though they are often constructed of other substances to avoid electrolysis and corrosion. Polyethylene is a common material for the construction of RV water tanks, though other plastics can also be used.
Some recreational vehicles only have one waste water tank, though it is common to have two. Having separate RV water tanks to drain the toilet and sinks can allow one to fill up and the other to still be used. In some cases, it is also safe to drain the gray water from the sinks and shower when away from a dump site. Many areas have laws against this practice, though some do not. These waste water tanks are typically mounted underneath the RV or in a side compartment and are usually emptied via knife valves.
Portable secondary RV water tanks also exist for both fresh and waste water. An extra fresh water tank can facilitate longer camping trips away from utilities, though that extra water has to go somewhere. A portable waste water tank can be used to store excess sewage when camping away from utilities, though it can also be used as part of the dumping process. In this case the waste water tanks in a recreational vehicle may be emptied into the portable container, which can then be brought to a dump station while the RV itself remains stationary.
Many recreational vehicles also use hot water tanks. These are usually metal pressure vessels that can contain heated water for various cleaning purposes. Just like a hot water tank in a home or business, these units sit in-line between the water source and the faucets. The tank typically contains some type of burner or heating element that can burn propane or use electricity to warm up water.