Sure, wind can cause a huge difference in mpg-so can rain. I'm surprised you couldn't see the trees move, but sometimes they just stay "bent in the wind" rather than sway.
I have also noticed that the Prius is a bit squirrely in wind; not quite dangerous, but noticeably less stable in wind of any sort.The mpg and the "funny feel' probably mean exactly what you think-head wind. Luck.Charlie
PS I've have driven thru your panhandle many times in other vehicles-talk about wind!!! I kinda wonder how the Prius will like that kind of wind!!