2012 Tundra Platinum issues

I found a great running 2012 tundra crew cab. 99K on it. Previous owner did some mods and not sure if they did something wrong or if this is just a problem that happens and can be fixed easily. The check engine light is on and keeps throwing the P0101 code. The mass air flow sensor has been replaced but still the code comes up. I noticed that even when I reset the Miles per gallon display on dash. Then drive it for 15-20 miles, it only shows that I'm getting 8-8.4 miles per gallon. I was told that the mass air flow sensor and other items that could be causing the code to come up, could also be causing bad mileage. Any suggestions?

 

Is there any mods to the air intake plumbing?