You want to find the trucks GCWR Gross Combined Weight Rating.
Then you want to load it up like your going camping, that means full tank of gas, all the people, fire wood and all the stuff would have on board for a trip, and go to the scales. Subtract scaled weight from the GCWR. That is the number you need to know. That number is the MAXIMUM weight you can tow before you exceed the manufacturers designed limits. From there start looking at trailers that have a GVWR number less that your available weight limit. All the rest is fluff, designed to sell you more than you can safely or comfortably handle.