Originally Posted by Lak
Why do so many makers of 5th wheels say warranty void if you use a gooseneck hitch?
Because a gooseneck adaptor puts a lot more leverage on the trailer frame under the overhang hear the kingpin. The frame of a 5er was designed to handle a fifth wheel hitch, but it has to be much stronger to safely handle a gooseneck hitch without bending.
My local welding/blacksmith shop can add a lot of steel to the trailer frame around the kingpin under the overhang to make it strong enough to safely handle a gooseneck hitch. He did that to his own 5er, and will do it for yours for a hefty price. If the underside of the overhang is finished with a cover to fully enclose the frame around the kingpin, then that cover has to be removed before the necessary welding can be done. On a lot of the better 5ers, that's almost impossible to get done without messing up the looks of the cover.