Wayne, I see this is your first post, so
I could go through a long technical discussion of the effect a gooseneck adapter has on a 5th wheel frame (if you're interested in the details, I'd suggest a search on the term "gooseneck adapter" in the search box at the top of the screen). In a nutshell, though, a gooseneck adapter is like bolting a cheater pipe to the pinbox - it's going to amplify the loading into the pinbox and is going to introduce torsional loads that are not present with a 5th wheel hitch due to the anti-rotation effect of the load plates on the hitch and pin box. 5th wheel frames, as a rule, are not designed for these loads - that's why a gooseneck trailer has all the gusseting in the crown above the gooseneck post. Can the 5th wheel frame survive this loading? Perhaps, and perhaps not. You'll undoubtedly have some individuals post their subjective positive experiences, but others have had frame failures. You pay your money and you take your chances.
If you're going to be purchasing a new 5th wheel, ask the RV manufacturer (NOT the RV dealer or gooseneck adapter seller) to confirm in writing that the use of a gooseneck adapter will not void your structural warranty. If the manufacturer is willing to do this, go for it. If not, the risk will be on you as many (most?) RV manufacturers will void the structural warranty if a gooseneck adapter is used.
There are alternates that will provide proper 5th wheel hitches with a clean bed floor when the 5th wheel hitch is not in use. One such system is the B&W Turnoverball gooseneck hitch used in conjunction with the B&W Companion 5th wheel hitch. More information on this system is available HERE
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