Well and RV water pump is just like it sounds,It ’ s a pump designed to work in a motor home,and RV, even a boat ,to move water from a storage tank,and provide water to showers,to galley sinks,even to help flush toilets. Whaleflo supply 12volt pump,and it ’ s gonna wire directly into the 12Volt system of the RV,and the difference in sizes.is based upon the difference in size of the RV.