My only complaint about the AWD switch is that it's location is poor and sometimes accessing the storage compartment in front of it, I push the button turning it on or off. I've learned to check it, especially if I fill up the tank and notice slightly less mpg. Not to the extent of the OP, but it does drop a bit.
As for the OP, if the engine is running fine, and no symptoms seem to be present, perhaps resetting the computer would help. It would then relearn to your driving habits and hopefully provide better mpg. Disconnect the negative battery terminal for about 30 minutes or more. Keep in mind that doing this will cause most or all of your settings to revert to default and you will have to reprogram radio stations and menu settings.
I'm assuming that you've checked the air filter and other normal maintenance items? One thing I have not seen much are gas mileage complaints. Only other thing is, does the transmission seem to be shifting ok? IE, it is not stuck in a lower gear causing the RPMs to run high?