That actually did happen to me as well. So the best way is, fill your tank to the brim..... Like ultimate brim.... Reset the meter and ride the bike for day 80 to 100 Kms. Then fill it again to the brim and get the actual average. That will tell you that if the average then drastically falls, then this is the only issue with you. With a medium twist and changing gears at around 4k RPM, it easily does 40-42 km per liter within the city even bro. Once it gave me even 47 kms to a liter. Even if it's 40km, multiply by 14.... That's 560 Kms to a tank. If you are getting 300 Kms, then your bike is doing 21 km per liter. That's IMPOSSIBLE. Bro my Mira does 22 to 25 km per liter in the city...lol
Anyways, for your satisfaction, get it tuned. Fill it. Ride. Refill. Then find the thief. Simple