GTWR 1980
Was this number derived from actual scale weight not hitched to the truck? It does
not appear to be consistent with the rear axle weights before and after being hitched that you provided. For example 6420-3260 is 2980 pounds. Is there other stuff in the rear of your truck and what does the hitch weigh and was it part of the first weigh without the fifth attached?
Also if the math is working out correctly your trailer has 9520 pounds axles weight
plus the tongue weight as weighed would be around 12,500 minus whatever the hitch weighs (around 200-250 pounds). Where did the 1980 tongue weight number come from? If it is a brochure or dry weight number don't believe it.