Yes, AC will most definitely affect your mileage, but just as you say it shouldn't be that much. You didn't say, but I assume it used to be higher.
Have you changed your little 12V battery? They slowly go bad, just like all batteries, with the result that the Prius tries, hard, to charge it up all the time. That might be something to consider - it is used primarily to click on the big relays to the high voltage battery, but that's about all so it isn't like the battery in other cars that you know is going bad because the starter will barely turn over. I know I saw the same big mileage drop at about the same time you are and replacing that small battery took care of most of it.
Again, you didn't say, but new tires will also hurt although to do so for a full year seems unlikely.
The only other thing is to take it to a shop for a complete check-over. A tune up on the engine (new plugs, air cleaner, etc.) and to look for anything else.