When I bought my truck it came with a auxiliary diesel mounted in the bed of the truck. It has a pump and electric fuel valve to transfer the fuel and is larger then my main fuel tank. My question is ...
We have one and love it. It is legal in all states. Ours is a 45 gallon under cover tank. We have a ReTrax rolltop cover and the tank is made to go under it. Works great. You might want to take a look at Aluminum Truck Tanks
The system works great. Pump feeds through a "T" into the factory fuel neck and into the factory tank. Fuel gauge is always accurate.