What your missing is trucks out here on the road from a legal standpoint don't carry weight determined by the vehicles GVWR nor a number on the tire placard. DOT isn't concerned with either number for determining a overloaded vehicle.
This is where RAWR/tire load rating come to play.
The tire placard became mandatory in the mid '00s era so many older trucks may not have one.
Which brings up the question what did a truck owner use to determine how much load the truck could carry before the tire placard. Then.... just as now we used the trucks GAWRs and RAWR in particular as it will carry 99 percent if not all of a trailers hitch weight.
Example;
my 2500 Dodge/Cummins has a 6000 RAWR and weighs 2920 lbs with the 5th wheel hitch which leaves my truck with a 3080 lb payload.
Check out GM online ordering guide which has a weights calculator that figures how much each trucks GAWR and gross weights with selected options......
http://www.gmfleetorderguide.com/
My reply above is simply to answer the OP question on what he is missing.