Every state has different requirements for reimbursement of use of a privately owned vehicle (POV)during work hours and prior and after work hours.  Regular employees do not get reimbursed usually for gas expenses and/or mileage when going from their home to their job.  It appears you are being reimbursed for mileage from the home station to the hotel but the employers treats your mileage from your hotel to your job site as if you are leaving from home to your job.  Contact the GSA.  The GSA usually has the POV rules and you can determine if your employer usually the GSA guidelines.  Many states and other government entities do indeed use the GSA guidelines.