auxiliary fuel tank for diesel truck

With gas most have the fuel pump in the tank. the fuel is also returned to the tank via a return line. So when installing two tanks in a gas vehicle you don't use a crossover. You would install a fuel pump in and return line in the new extra tank, or at least I would. This would be a lot of work though.

You could use a crossover or transfer between the two if you wanted to, and that would be the easiest way.