Quoted directly from the Cornell source:
"(d) Gravity or syphon feed prohibited. A fuel
system must not supply fuel by gravity or syphon feed directly to the carburetor or injector."
Nowhere does it say it can't gravity feed into another tank. Just not into the carb or injector.
The only thing I see that REMOTELY resembles gravity feed to another tank being not allowed is the following:
"(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 Except for diesel fuel tanks, the fittings through which fuel is withdrawn from a fuel tank must be located above the normal level of fuel in the tank when the tank is full."
In which case, it means you can install a pump to fill your stock tank. Nowhere does it say it cannot be electric. The only statement about this fuel transfer is the following:
"(e)
Selection control valve location. If a fuel system includes a selection control valve which is operable by the driver to regulate the flow of fuel from two or more fuel tanks, the valve must be installed so that either -
(1) The driver may operate it while watching the roadway and without leaving his/her driving position; or
(2) The driver must stop the vehicle and leave his/her seat in order to operate the valve."
EVEN THEN, there is this tidbit:
"(4)
Drains and bottom fittings. (i) Drains or other bottom fittings must not extend more than three-fourths of an inch below the lowest part of the fuel tank or sump.
(ii) Drains or other bottom fittings must be protected against damage from impact.
(iii) If a fuel tank has drains the drain fittings must permit substantially complete drainage of the tank.
(iv) Drains or other bottom fittings must be installed in a flange or spud designed to accommodate it."
Which, while this pushes the definition of 'drain,' means if you had a petcock valve to drain the auxiliary tank that just HAPPENS to drain it into your primary tank, it could be allowable.
I could find NOWHERE in any of these that state you cannot plumb your auxiliary tank into your primary tank's filler neck.