For anyone who still wants to know the difference between Gross Vehicle Weight Rating (CVWR) and Gross Combination Weight Rating,
Here it is from the Feds themselves:
Gross combination weight rating (GCWR) means the value specified by the manufacturer as the loaded weight of a combination (articulated) vehicle. In the absence of a value specified by the manufacturer, GCWR will be determined by adding the GVWR of the power unit and the total weight of the towed unit and any load thereon.
Gross vehicle weight rating (GVWR) means the value specified by the manufacturer as the loaded weight of a single vehicle.