Absent a state law specifically addressed at the employer, the employer can out his/her equipment with recording devices. This issue is less where the devices are and more who is the driver. If the regular driver of this vehicle is a member of a protected class there may be a problem. A protected class is the way the law determines discrimination. Current Federal discrimination bases are: age, sex, race, color, nationality, disability, religion, pregnancy, marital or military status. (see www.eeoc.gov) You state also enforces these discrimination bases and may add more of its own. Many states are now adopting 'sexual orientation' as a base of discrimination.
So, for example, if the usual driver of this truck in a minority, then he/she may be able to file a discrimination complaint against the employer. However, before entering into costly and time consuming litigation that employee should consult with a local employment attorney.