Inevitably I'd fill up at the beginning of that long trip and gas would be cheaper the whole rest of the way.
I've found that I gotta stop for leg stretching breaks, or breaks of some sort, before the car needs filling.
Anyway, I've plumbed into a gas can in the trunk before... then I upgraded to a snowmobile gas tank (had a gauge on it, and was much bigger). That was just on old carb'd stuff. I see two ways to go about it: transfer pump into the primary tank, or a separate but equal tank w/pump- heck, that might not even need a valve or check valve if the pump assys have check valves in them. I think the second method is how F150 dual tanks are rigged, but I've never studied them.
I'd love to have dual tanks on my car. I could keep E85 in one and E10 in the other. And/or have a primary tank sized for reasonable fill intervals for around town use- say, a 5 gallon primary tank with a 10 gallon secondary tank that I'd fill for long trips; that would maybe save 50 lbs from having a full 15 gallon tank, and the small tank wouldn't have all that sloshing going on all the time.
Maybe the secondary tank could have a quick-connect like outboard boat motors have, so that it's kept in the garage until needed, then bungeed down in the trunk when wanted.