Are holding tanks necessary and what is their purpose? What is the size of these tanks?

Holding tanks are used to store fresh water, grey water, and black water. Black water refers to wastewater from the toilet, while grey water is typically soapy or dirty water from other drains. Fresh water tanks hold clean water for use in the unit. While not all tanks are mandatory, a fresh water tank is recommended if you'll be traveling or stationed in an area without a water source. Grey water tanks may not be necessary if you can safely drain water directly into the environment, but you should check local regulations. Black water tanks can be bypassed with methods like composting, dry-flush, or direct sewer drain.