For the lowdown on what is required you have to go to the regulations.
FMCSR 393.11 is the DOT regulation that says you have to have the retroreflective sheeting, and for the specific requirements it sends you to Federal Motor Vehicle Safety Standard No. 108 (49 CFR 571.108), S5.7, Conspicuity systems. That is the requirements for manufacturers of Truck Tractors made after July 1, 1997.