10 kms for 300 pesos of gas will equate to nearly a liter for every kilometer ang consumption ng kotse. at the rate, it cannot be what both mechanic had told you. not even a F1 car would consume at that rate.
check first for the following:
1. for leak in the tank and/or in the fuel line.
2. if your gas sensor is still working fine.
3. did the gas station really gave you a P300 worth of gas?