My understanding was that the auxiliary tanks where you pump the fuel into the aux tank, and then manually pump the fuel from the aux tank into the vehicle tank, *are* legal for gasoline. You pretty much have to stop, get out, and do the pumping while stationary (unless you like to live dangerously). Still beats the heck out of having to pull into a truck stop, or having to juggle your fueling stops because of long stretches without anything resembling civilization + an engine that gets single-digit fuel economy.
What's not legal (AFAIK) is the kind you see on diesel TVs, where they have *auto* transfer pumps to ship the fuel from the aux tank to the vehicle tank, while it's running. The impression I got was that the DOT won't sign off on such aftermarket devices for gasoline engines, due to the explosion hazards compared to diesel.