Charge for used fuel and gas is an issue between you and the renter only. As a guiding principle, renter is to return the vehicle with fuel and gas as received from you.
It is recommended, that the vehicle is fully fueled before handing over to the renter, that all gas bottles are full, and that you jointly note the mileage while inspecting the vehicle.
For your own expenses for gas and possibly exterior cleaning, you may consider adding a handling fee to your rental price. That can easily be done in step 7 when you create you vehicle listing.