Need help fast!

I know that people are constantly opening threads about their mpg and it gets annoying because there are so many factors but situation is different. Please help me. I have an 09' double cab trd sport and just broke 20k miles last week. I have been running mobile 1 synthetic since day 1, I have all stock tires/rims, no engine mods other than my aftermarket muffler but that has also been there since the beginning. My driving habits have not changed and I still maintain about 70% highway 30% city driving. Never punch it hard, always doing the speed limit, and always running 87 octane. Nothing has changed in over a year of driving. I just did my oil change less than 1k miles ago. Tire pressure looks good but I haven't actually checked it. I used to average 16-20mpg depending on driving habits... All of a sudden I'm lucky to get 12 and NOTHING HAS CHANGED! I just threw a can of seafoam in the tank and some in the crankcase to maybe clean the injectors but I won't know for a few days the results. Any ideas please help. Still under full warranty do you think the dealer will help? Thanks guys

 

Has anything changed?
(EDIT) Since you posted.

 

Bad gas? I would try going to get gas from different places or higher octane to see if that makes a difference.

 

I have switched to 91 octane.. i now get approx 1.5 mpg better, and save 1 cent per mile.. which is alot if you add it up overtime..(im also the guy that picks up pennies on the ground wherever i see them ... haha jk.. but seriously.....)