The CAT scale works very well for your situation as it provides the weight for the truck with the load on the hitch from the trailer tongue and the weight of the trailer itself. There are two gross weight concerns with the first being the total weight of the truck with passengers and cargo and the load from the trailer tongue or kingpin. Separate is the gross weight for the trailer when fully loaded.
My truck has a GVWR of 11,000 lbs and this applies to the load on the first and second scales. The trailer axles are on the third scale and this load needs to be less than the combined weight rating for the trailer from its manufacturer.
Another important number is the GCWR (gross combined weight rating) which refers to the maximum load in total of the truck itself with passenger, truck cargo in the bed, hitch load from a trailer, total weight of the trailer with its load. For my 2011 2500HD the GCWR is 24,500 lbs total for truck and trailer and if I go to a CAT scale I can quickly confirm the Gross Combined Weight of the truck and trailer.
I do a weigh in at the local CAT scale to have a starting point by knowing the weight of the truck at its front and rear wheels and the weigh to the trailer with truck and trailer empty. With my truck I have also found that only 20% of the load in the bed or on the hitch will affect the front wheels so if I add a 4000 lb load in the bed the front wheels need to support 800 lbs of that load and the rear wheels will be supporting 3200 lbs. (or 1600 lbs supported by each tire).
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