Really Bad Gas

I have an '07 Prius. I had been using Exxon and was satisfied with 45-50 mpg. Then a funny thing happened. New gas at Exxon a couple of weeks ago, and it went to 35 mpg. I went to Toyota and told them there must be something wrong, maybe one spark plug failed? I had needed a checkup anyway, and they did it. They found nothing wrong. I thought "well here we go again with them not finding what's the matter". They told me there's a fuel addition gas companies use in the colder weather. It should take down my mpg maybe 5%. Well I got my Exxon gas again and again, and it stayed at about 35 mpg.
Then on a trip I needed gas. Amoco was there and I filled it up. The mpg returned to 50 mpg.
I figure I got bad gas from the Exxon stations nearby my house. And I figure they gyped me out of about $30 for 3 fillups. No wonder their chairmen retire with a package of $600 millions.
Where do I get my refund?