Your Nissan dealer should have all the information you need to determine the tow rating for the truck. It's THEIR warranty that's involved in the rating so only THEY can determine where they draw the line. The SAFETY numbers you need to keep in mind are the trucks GVWR and the trucks axle ratings (those numbers can be found on the door plate of every vehicle and are required by law). When the fully loaded trailer is hitched up your truck must weigh under it's GVWR and all axles must be under their axle ratings. Calculating your trucks "payload" is pretty easy. It is the weight of the truck (taken at any time) subtracted from the GVWR. THAT is how much weight your truck can safely handle and it includes weight from all sources. That's passengers, driver, cargo, optional equipment, trailer tongue weight etc.
Good luck / Skip