Well In my drive down to TX I am happy to report that my first tank (MN Gas) got me 23MPG and my second tank in IA got me 26.6 MPG and my Last tank in KS got me 28MPG. So as I thought something probably had to do with crappy MN gas. She's got some Chevron in it now and seems to be loving it. too bad I can't take a number of tanks up with me.

 

I think it has less to do with "Minnesota gas" than it does with Minnesota WEATHER. Notice that, as you went south, your mileage increased. This could have been due to different driving conditions (I know Kansas is as flat as a... well, it's flat), but I suspect it has more to do with outside air temperature.
Here in Maine, in mid-winter, my Maxi gets sucky gas mileage (18MPG). Peaks out during temperate weather (around 75 degrees) (24-26MPG) and then doesn't get much better in hotter weather. Humidity plays in there, too.
I had a design engineer (my brother) explain to me why this happens; density of the air, engine parts not warming up, etc. Different quality of gas may make a minor difference in mileage (we're talking 10ths of a MPG), but probably not 5 MPG (unless the Minnesota dealers are mixing it with water).

/just my 2¢ worth

 

colder = denser = more air/volume = more fuel (and power, although thats not the issue here)

 

your car may be running better because of the long drive. Sometimes all it takes for my cars to achieve good performance again is to leave the city for a several hundred mile drive and they run much better. Also, is there an altitude different between the places? Altitude plays a big role. The cars are much worse driving through the mountains near my cabin, compared to the lower altitude of the valley.