Auxiliary Tanks are plumbed right into your stock factory tank. This is sometimes accomplished with the aid of a small 12v pump or gravity fed. Tanks almost always have a shut off valve so you can stop the flow of fuel if necessary or if wanted. Since a lot of the connections will be made under the trucks cargo floor you’ll want to make sure your tank and lines are sealed properly and free of leaks. With an electrical pump in your system, you would typically just keep an eye on your factory fuel gauge and when you wanted to refill, you would just flip the switch you would have mounted inside your truck and refill the tank as you roll down the road. If you use a gravity fill system, gravity will simply replace fuel into your factory tank as your engine consumes fuel. Because of the danger that you could forget to turn your electric pump off while filling and consequently overfill your tank, electric assisted pumps should have a return line installed to direct overfill back into the auxiliary tank. [links]