Fresh water and waste tank capacity are two of the most important factors when choosing a restroom trailer. A restroom trailer needs enough fresh water for flushing toilets and handwashing sinks, and enough waste tank capacity to collect used water and toilet waste during the event or work period. This guide explains how restroom trailer water systems work, what buyers should check before purchasing, and how to choose the right tank setup for weddings, festivals, construction sites, outdoor venues and rental businesses.