Use the trailer GVWR which is the max it can weigh loaded. The tow vehicle must be able to pull that. On a 5th, the trailer hitch weight is born by the truck bed payload capability. Include the weight of the 5th wheel hitch. Also take into consideration the truck GVWR. Strongly suggest having at least a 10-25% fudge factor.
Read the vehicle specs carefully and don't depend on a salesperson knowing what they are talking about.
There are lots of You-Tube videos of what happens otherwise.
Something else to consider when looking at trailers and trucks are the tires. Suggest
https://www.rvtiresafety.net/