This subject has been discussed several times. I guess I am a bit different than others. My 45gal tank is connected to a valve that switches between the OEM 38 gal and the Aux in the bed by a switch in the cab of the truck. It also is connected to the fuel gage so I can see how much fuel is in each tank.
When I get low on one tank I just flip the toggle switch I start using the other tank. The fuel gage takes a bout 20min to show the fuel level if I am driving. When I start the truck it will tell me the level of the tank I am switched to with no delay.
They will send you all the installation instructions and the wiring kit to switch between tanks. You can go online and see the installation instructions at the link below.
American Tank and Tool Box, Inc.
930 North Highway 377
Pilot Point, Texas 76258
BC