You are forgetting something Idasho . The factory bed mounts are rubber cushioned mounts. Your hitch pictured lock's the bed isolators out.
Ending up with a solid ridged mounted bed. NO factory WARRENTEE. Ford Chevy or Chrysler. As do most old style 5th wheel hitches.
Most rv owners don't have a hoist large enough to lift a bed off a truck. I do . And it sounds like you do ( or have 4 or 5 good burly friends with nothing better to do) but most don't.
And depending on the truck you may have to drop the tanks, which on some trucks are suspended from the bed. It' just not as simple as you try to make it seem.
If you pay to have it done , get ready for a large bill. No reputable shop will pull a truck bed for under a grand. That's not including the hitch , bed modifications and wiring just the labor to remove and reinstall the bed.
As to the trailer manufacturers and there warrantee. Who knows?
If the trailer wasn't designed with allowances for the added torque at the neck , I'd shy away from the conversion.
As I stated earlier my 5th wheel was designed and built as a gooseneck.
And no Idashow the over bar 5th wheel hitch design actually spreads the torque to the truck frame better than the gooseneck you picture. It has a larger foot print and a much wider bolt pattern there by reducing the torque by spreading it over a larger aria..........