It's hard to tell with the information that you provided. Calculating based on the 15,000 GVWR of the 5th wheel is a good plan as it helps you prepare for a worst case scenario. Typically 20% - 25% of that weight is going to be carried by the truck in the form of pin weight (3,000 - 3,750). There are fringe cases where the pin weight percentage is less (like my camper- at 17-18%) but, imo, it's better to be conservative.
Most trucks run out of payload long before hitting their other ratings. The payload is what the truck was designed to carry (itself, driver, all passengers, stuff in the truck, hitch, stuff in the bed, add-ons to the truck, etc.). You will have a sticker that tells you that maximum amount of cargo and occupants can be on your doorjamb - this is for your empty truck - everyone and everything decreases that number.
You need to estimate all of those things and then ensure that the pin weight from above can still be carried by what is left over.
Here's a write-up that I did for these kinds of questions:
What Can I Tow?Hope it helps!