If it is a solid 14,000 pounds dry, and add over 1,600 pounds of motorcyles, you are over 15,600. A FW that big you will probably add closer to 2,000 pounds of food, clothing, tools, kitchen ware, towels, sheets,....on and on....
If you haul auxiliary fuel you may be over 18,000 pretty quick. I assume it is a triple axle. Since the guy you bought it from probably was not telling the truth about being able to pull it with his truck, check to see if there are G-rated tires on it (or higher). Believe it or not, it may have came out of the factory with tires not rated for all the weight you will end up having on it.
It is was me, before deciding on what truck to start my search for, I would hook up and take it across the scales with the motorcycles loaded. Even if you don't have all your stuff in it yet that will put you in the general ballpark on weights. It will be $10.00 well spent.