Well, here's another update.

So far, I've driven around 255 miles and the fuel gauge is just below the quarter tank mark. At this point, it appears that I'm getting about 15mpg, as I'm burning a quarter tank every 75 miles. That would mean I'd get about 300 miles per tank. I'm going to try filling up at other stations to see if I get a better result after going through this tank, though I'm wondering if too much ethanol in fuel can have such a huge effect on fuel economy (20% drop!). The only place I can get gas without ethanol gas is at the marina, so I'll try to see if I can fill up a tank there to rule out the ethanol.