2003 Acura MDX 33% reduction in fuel economy

My well maintained Acura MDX 03' recently traveled through Canada and the Yukon Territory. Although my manual states 91 octane minimum, I was forced to use whatever was available through these very remote, areas. (Unleaded with who knows what in it). Prior to the drive I was getting 17 -18 MPG city driving; now the best I ever see on my trip computer is 11/12 MPG. I am driving in colder conditions (30F), but I can't imagine the cold air alone is the cause for a 33% reduction in fuel economy! I took it into the dealership who scoffed at me when I mentioned it might be fouling of the spark plugs, and told me 12 MPG sounds about right. (Not according to my manual). My manual did mention something about using gas in Canada and negative side effects. I have no knocking sounds and I have had the air filter replaced, what else can it be? What can I have looked at when I take it in? Also, where I live, the best gas I can get is 90 octane, not 91, is this ok?