Three tanks that are used in an RV to aid in the travel experience and establish the basic sovereign and autonomous standard that makes RV traveling such a freeing exercise are the fresh tanks and two waste tanks. If there is a fourth tank it's usually the hot water storage that is responsible for heating up water for washing the dishes, taking a nice hot shower or creating heated water for outdoor clean up.
The biggest tank of all of these is usually the one that is used for the fresh water supply. The waste tanks together will typically be larger than the single water holding unit. The sinks and the shower area will drain out into one of the tanks know as the gray tank while the toilet water will drain into the black water storage. Water tanks for RVs are usually made of ABS plastic and are tough but light enough to be able to be carried around everywhere the trailer goes without weighing it down too much.
The majority of travel trailers, campers and motorhomes make the most use of the top three water tanks mentioned above, the fresh tank, gray and black water tank. The fresh one is much of the time used for potable water, but this doesn't have to be the case and may not be something that is drinkable. Sometimes these fresh tanks are made out of metal except this can cause a problem with corrosion and for this reason plastic is very popular. These units can be manufactured out of polyethylene although other types of plastics can be used as well.
While many RVs have two waste tanks others may only have one. The problem with only a single is that if it fills up all the way then there is nowhere else for the toilet and sink waste to go. If there are two tanks then when one is full the other may still be used although toilet water usually won't go into any tank except for the black tank. At some campgrounds the gray waste tank can be dumped away from the designated dump site although this is usually frowned upon. These tanks are normally mounted underneath the camper or possibly even in a side bay.
These are often drained through special valves. Portable waste units can add to the capacity of the RV and allow for more water to be drained on longer excursions without needing to find a designated dumping site right away and can extend time between dumping. The hot water tank will often be placed between the fresh storage and faucet areas and will require a propane or electrical heater to warm up the water. Learn as much as possible about RV water tanks before using them to make it easier to maintain them and troubleshoot them if there is a problem once out on the road.