With the information given, we can't do better than a good guestimate.

Fuel consumption comes mostly from three components: Rolling resistance (linear with speed), the energy used to run the motor (square of rpm), and wind resistance (fourth power of speed). Each of these has a constant which depends on the weight, on the construction of the engine, and the shape of the car. Note that the distance also grows linear with speed, so going faster doesn't increase the energy needed to overcome the rolling resistance over a mile, just over a second or minute.

The fuel consumption was given as 7.2 ltr / 100km. This is shared between the three components. 7.2 is a lifetime average. If you drive to save fuel (hyper-miling), you can't reduce the rolling resistance, but wind resistance and engine resistance by driving slow and in a high gear. The fuel used for the rolling resistance could be approximated by taking the fuel consumption when the driver tries their best to save fuel (minimising engine and wind resistance), and then subtracting some more because there is still engine and wind resistance left. With 7.2 ltr average, 5.5 ltr / 100 km is probably achievable, and I would estimate 4 ltr / 100 km for rolling resistance.

The car weighs 1200 kg. A full tank weighs 37.5 kg. On average, the fuel in the car will weigh about 20 kg (because nobody empties their fuel tank completely). Additional weight for driver and passengers may be 100kg on average (people mostly drive alone, but not always). So the total weight on average is 1320 kg, fuel weight on average is 20 kg, and with 300,000 km I'd estimate the fuel spent to carry the fuel is

(4.0 / 100) * 300,000 * (20 / 1320) = 12,000 / 66 = 182 liter.

So the guesswork involved was: How much of the fuel consumption is weight dependent? (Less than the total fuel consumption). What percentage of the total weight is fuel (need to take into account weight of car + passengers + stuff in the car).