When you use a trailer, the max Gross Combination Vehicle Weight (GCVW) determines how much you can carry. This number will be on the same data plate that had GVW and axle weights.
Subtract the empty weights of the truck and trailer from GCVW to determine how much weight you can legally carry when the truck is combined with a trailer.
You can carry much more weight; that's why there we see so many pickups pulling trailers these days.