Octane will have absolutely no effect on mpg when drive non-aggressively or aggressively for that matter. Minor effects yes but not an average of 3 gallons. A couple things though
GIven its low bottom end torque, if you are driving it too slowly to get up to speed, it actually uses more gas than if you have a semi-aggressive departure from a standstill to speed. It takes more work to get to speed in the lower gears at lower rpms.
Second, given the big storm, I would pull your air filter and see if it is clogged or dirty. That is one thing that really can knock 3 mpg off easily.
Last, sure, there is bad tanks and quality, but its not the octane but rather the gas itself may be bad which is possible given all the shortages etc.