That's not actually how it works. The 10k for the trailer is what the trailer is rated for with cargo. It adds zero to the trucks gvw. The 6,800 has nothing to do with trailering. That's the loaded weight of the truck (truck, cargo, and passengers). Your trailering limit is listed separately and will be different depending on how the truck is optioned and the type of hitch and trailer. At 7,700 for the trailer and cargo you were most likely overloaded or at the limit if the truck was max equipped. Late 80's half tons aren't rated for much. 16,800 is modern diesel territory.