I think what Duffster was saying is "here" in the law not here in that state. Almost all motor carrier laws are federal mandates so they are pretty much the same from state to state.
Gross vehicle weight rating (GVWR) is defined in South Carolina Code of Law, Section 56-1-2030, item 16 as the weight or the value specified by the manufacturer as the maximum loaded weight of a single or a combination vehicle. The gross vehicle weight rating of a combination vehicle (commonly referred to as the "gross combination weight rating") is the gross vehicle weight rating of the power unit plus the gross vehicle weight rating of a towed unit.
In a nutshell - the axle rating is not how GVWR is determined.