One thing you need to understand here is that this forum comprises only a small percentage of all RVers out there, and that absence of anecdotal evidence is not evidence of anecdotal absence.
You are perfectly free to go ahead and try it for yourself and see if it works for you. If you twist the frame on your fifth wheel, you have your answer.
If you understand basic physics and how cheaply RVs are manufactured, you would have concerns about the extra leverage one of those adapters puts on what amounts to a barely-adequate frame.
You can find plenty of anecdotal evidence of people successfully using generic gooseneck adapters successfully, which leads me to conclude that the possibility of damage is real, but minor. However it is not a risk I would be willing to take with a $50,000 5th wheel trailer, knowing that warranty would not cover the damage, should things not work out perfectly.
There are alternatives such as the Reese Goosebox, which is designed to adapt a gooseneck to a 5th wheel, and approved by Lippert, who manufactures a good portion of the frames for these trailers, or the Andersen Ultimate series of 5th wheel hitches, which go at it from a different angle. Lippert does not recommend the use of a conventional gooseneck adapter with their frames.
