Gasoline Transfer Tank.

Sport45 wrote:
The law essentially says a gasoline tank cannot withdraw fuel from a connection below the fuel level.

49 CFR 393.67 (c) wrote:
5. Fuel withdrawal fittings. Except for diesel fuel tanks, the fittings through which fuel is withdrawn from a fuel tank must be located above the normal level of fuel in the tank when the tank is full.

I take this to mean an in-bed auxiliary tank with top connections can pump or siphon to a vehicle main tank without issue.

If the 2 tanks are connected together via a gravity feed tube, then the normal fuel level is the level in the aux tank, far above the top of the truck's main tank.

The truck would have to draw directly from the aux tank, not from it's main tank (and thus the main tank becomes the aux tank), for the law to be (partially) satisfied. There's still the issue of the connection at the bottom of the in-bed tank.

Pumping the gas out of the main tank via a pump in the in-bed tank would likely not be possible, because the pump in the in-bed tank likely would not be able to draw up the fuel from the truck's main tank, through the tube connecting them together.

That explains the gravity feed issue.

As far as pumping from the top of the aux tank, that is legal, but not sure if it is legal while driving down the road. You can have an aux gas tank with a pump nozzle on it that you pump into the fill neck of the main tank, as needed, when pulled off the road. That way the 2 tanks are not connected together in operation.

Or, you can add an additional gas tank under the truck at a similar level as the main tank and connect it to the main tank with a switching valve. This was common on older trucks (1960's - 90's) that came from the factory with dual tanks.