Sorry didn't see this.
Custom built hitch is rated to 500/5K but I wouldn't push it past the 300/4K I'm running now without upgrading the trailer suspension. Factory rep said the frame was good to 5-6K pull/stop but I've seen others have issues without reinforcing the pinbox area and front end. The 300 is well within the total GVWR of the fifth wheel when wet and ready for a week's outing and I'm 10K under the GCWR of the truck (HO/AISIN). I have about 700lbs of payload left with genset, hitch, pin weight, 50 gals of fuel, people and gear all loaded.
New factory (same or larger) GDRV models since 2019 are coming with tow rated hitches 300/3000 or 350/3500.