This is one of the real touchy questions. Some people use gooseneck adapters and they work fine, while others have experienced major trailer frame damage. The gooseneck extension acts as a lever against the 5W front overhang and some trailer frames aren't up to the extra stress. Cracks may appear where the front overhang of the 5W joins the main trailer body. Might be fine forever, or might crack the welds on the frame the first time out. Or the damage might be gradual and take years to become visible. I don't see how anyone can say in advance, before you even identify a particular trailer, whether yours would be ok or not. Chances are it would be ok, but there really aren't any guarantees.
RV and trailer frame manufacturers are going to be reluctant to ok using a hitch method they didn't design for, so may say "No" simply to protect themselves. I would still inquire, but would not be surprised to hear a "no" even though anecdotal reports say otherwise.