Interesting you should mention the sand. I found a small amount in the upper left corner as well. The filter didn't look that dirty, but was quite clogged with dust.
My normal fuel efficiency is about 28 mpg highway and 23 mpg city. Replacing the filter restored it to those levels. However, 300 miles/tank seems a little low -- but then you have a slightly larger engine than I do.
Also, I had been a consistent Z-Max user through last winter, and that has made a difference. Although the stuff does what is promised, there were recently some negative comments around it (probably started by jealous rivals), and I believe there is a thread here describing it. I plan to keep using it anyway, because it works for me.