Aux tanks raise a couple of interesting quandaries to consider. The $6.50 fuel is easy to avoid with a bit of planning. But at some point, you will need to refill the aux tank and OEM one. So when fill up time arrives, you may be paying $5 or $5.25 a gallon, depending where you are. I have considered adding an aux tank but have never been able to make the numbers work for me. Here in the Lower 48 I would like to have one just to be able to refuel once a day.
I have a 34 gallon OEM tank and have never had a problem finding fuel when I need it. Sometimes I will refuel even if only down 1/4 of a tank on the Alaska Hwy. anytime we stop to eat, I will usually refuel, regardless of fuel level.