We tow a 36' triple slide 5th wheel with a 13,500 lb GVWR and a 10,900 lb dry weight. We are right at the 13,500 lb rating with empty tanks loaded and headed down the road. This is because since our 5ver left the factory (where its dry weight was recorded), we've added a 2nd A/C, a washer/dryer and all our "stuff". Therefore, from our experience, the safe thing to do for many (not all) 5vers is to use the GVWR as the actual loaded weight for your thumbnail calculations.
Our 5ver carries about 20% of its weight on the pin. Therefore, we're running 10,800 lbs on the 5ver's axles and 2,700 lbs pin weight. Loaded and headed down the road, we're at 21,180 GCW (21,500 GCWR) and 10,380 GVW on the truck (10,500 GVWR).
Rusty