Ideas Needed: Why sudden drop in mpg?

While on a rv trip in july, in nj I refueled our HHR towed at a sunoco on the garden state parkway I was getting 31 mpg before refueling (normal). Once I refueled mpg dropped to 24 mpg . Drivin habits and the terrain the same. Brought it back to nc and still 24 mpg. Refueled a week later in charlotte area and same 24 mpg. A week later refuled in mooresville, nc and back to 32 mpg. Sunoco and shell, and sheetz gas. Weather the same rain some sun at times and very humid. The research octane rating at tbe pump sucks. They have no ideas of mixing ethanol properly, I think its manipulated by who's running the mixing valve at the refinery or who's asleep at the valve. Ther probaly sneaking in loads of E15 with the E10 at the pumps for testing resuts for future.