Except as otherwise provided in this Code, all owners of vehicles of the second division which are designed, equipped or used for carrying freight, goods, wares, merchandise, or for use as living quarters; and all owners of vehicles of the first division which have been remodelled and are being used for such purposes; and all owners of motor vehicles operated as truck tractors to the weights of which are added to the gross weights of semitrailers with their maximum loads when drawn by such truck tractors; and all owners of vehicles of the second division which are used for carrying more than 10 persons, shall pay to the Secretary of State for each registration year, for the use of the public highways of this State, a registration fee of $10 for each such vehicle, which shall be collected as part of the flat weight tax assessed under Section 3-815 of this Code. A self-propelled vehicle operated as a truck tractor and one semitrailer or a combination of a truck tractor and semitrailer drawing a trailer or a semitrailer converted to a trailer through use of an auxiliary axle or any combination of apportioned vehicles shall be considered as one vehicle in computing the flat weight taxes under Section 3-815.