To throw a monkey wrench into this discussion.......my truck has a Cody gooseneck hitch and they have an optional air bag coupler. These things are VERY heavy duty rated at 30k towing and 6k pin weight.
So not wanting to spend a ton of $$$$ on a new 5th wheel air hitch I called Heartland and spoke with 2 different reps. They both basically said that Heatland does not "RECOMMEND" gooseneck hitches. They said the problem you may have if a warranty issue comes up in regards to the area around the king pin box is with the frame mfg. who is Lippert.
I also spoke with a regional sales rep who said that he deals with a lot of rancher/farmer folks who have goosenech hitch's in their trucks to haul livestock. He said that they usually have heavy duty gooseneck couplers built to replace the king pin box and have had no adverse effects on the Heartland trailers.
He said that he does not like some of the gooseneck "adapters" that clamp to the king pin as that puts a lot of stress and strain on the king pin box. He said that he "wouldn't have a problem" with a well thought out, well built gooseneck hitch that replaced the entire extended king pin hitch unit.
Anyway Jumpshot, I would definately call Heartland and Lippert and get anything in writing (which you probably won't) before installing a gooseneck. All of these rep's will have amnesia if something goes haywire and warranty work is needed.....Dave