Two factors are involved.
Tow rating and payload rating. Tow rating is pretty clear.
Payload rating is variable depending on your truck or beliefs. CCC vs RAWR
The truck has a CCC sticker on the door jamb. That tells you how much weight the truck can carry, i.e pin weight, camper weight, tongue weight.
While you may be able to tow a 15,000 5th wheel, you may not have the payload capacity for the pin weight, 5th hitch and passengers in the truck.
Same for a travel trailers tongue weight.
Some use the door sticker as gospel and some use the RAWR.
I won't get into a discussion on the merits of either.