I would leave fuel costs on page 1 of Schedule C as operating expenses since you are not reselling the fuel to the customers. Other standard operating excpenses would be the same. Reserve COGS for items you turn into a profit center - items, not services. You could make Labor OCGS by deducting payroll but to my mind that gets messy.

If you want to determine profitability and what should be charged then that is a different story and different calculations. You can add fuel costs as we as labor and materials to any customer job. Then you can run P&L by customer and/or job.  Only charge what is applicable to the actual jobs - not what it takes to keep the lights on - you would be open for business even on days when there is no business.

In the overall to determine if pricing is adequate you would even have to include depreciation on the trucks as well.  This is a good exercise for off the books use of excel. Plug in billable hours over time, depreciation, taxes, insurance, interest, debt payment to determine actual cost per hour billed. There is more involved in this but consider where yoiu take those trucks for service. Probably $80-$100 per hour flat rate shop time but chances are the average tech ony earns $15-$25 per that same hour.