Thanks for all the great input here. I have a related question. I currently have an in-bed toolbox/tank which holds 45 gallons. It has a valve which switches between the factory and aux tank. It also switches the sending unit between the two tanks so the aftermarket tank is shown on the fuel guage when running from the aux tank.

I saw references to two sending units and two tank readings. This implies there is another set of wires to connect to another sending unit. Where and what color are these wires? Is there any programming to allow a second fuel level to display in the menus?