2006 Ford F-150 gas type

I bought a used 2006 F-150 pickup with a 4.2 L engine last September. The original owner (I am the 2nd owner) told me twice over the phone that he was getting 17-18 mpg city and 25-27 highway. He was using BP premium grade. September, I was getting 15 city and it has been downhill every since, even as low as 9.90 mpg city.
I was using cheap gas with alcohol added. I have since switched to non-alcohol 87 octane. Ford Motor Co. Recommends 87 BP non-alcohol. It had a rough idle sometimes then would clear up. The other day engine completely stalled twice but would start o.K. Do you think that by using non-alcohol BP brand is that important and could it clear up my problem?