Special purpose machines, either self-propelled or drawn as trailers or semitrailers, which derive no revenue from the transportation of persons or property, whose use of the highways is only incidental, and whose useful revenue producing service is performed at destinations in an area away from the traveled surface of an established open highway, and which carry no load other than their own weight, which cannot be divided for all practical purposes. This definition shall include a truck or truck tractor when used while drawing special mobilized machinery but this shall not be construed as exempting from license and registration the pulling unit truck or truck tractor as required by the motor vehicle license and registration.