I think you are confusing terms. The tongue weight is the weight the hitch "feels" so you don't want to exceed the hitch capacity (rating) and it is also the same weight that gets added to your carried cargo (people, luggage) that is part of the available payload/cargo weight. I think you were referring to it as hitch weight.
You can get the payload capacity by using the trucks GVWR (or look it up):
GVWR minus the actual weight of the truck (CAT scale)=payload.
Ex GVWR 7700 and empty weight of truck is 6,000
7700-6000=1700 available payload
That number (1,700) means the maximum amount of carried payload incl tongue weight of trailer (and hitch weight). You have to add up the weights of all people and cargo and the tongue weight of the trailer and the weight of the hitch (WD). Sometimes this number is posted on the yellow sticker on the door jam or sometimes it is in the owners manual.
The hitch weight you describe is the weight of the weight distributing hitch and hardware which can typically be 150 pounds depending on model.
Tongue weight is typically the trailer weight times 10%-20% (avg 15%) depending on how you distribute the load on the trailer.
Ex if the trailer is 6000 pounds
The typical tongue weight for a 6,000 travel trailer is 900 pounds.
thread on tongue weight:
So in this scenario you have 1,700 available cargo. Subtract for 2 people (180 pounds each) and 2 suitcase (50 pounds each) and hitch weight of 150 pounds
1700-360 (people)-100 (suitcase)-150 (hitch weight)=1090 pounds capacity left for trailer tongue weight.
1090-900 (tongue weight)=190 pounds under the max capacity (good).