I have already broken the frame around the pin box of the previous SOB trailer and I did some engineering design calculations and there is very little additional stresses we can add to the pin box. A Gooseneck can add over 200% the reactions at the pin box. No way I would even think of using a gooseneck on a pin box designed RV.
In addition we have heard of broken frames, why would someone take a chance.
Also I discovered the broken frame after I was towed (Emergency) by a gooseneck setup for a distance of 20 miles. The driver used his jakebrake instead of the trailer braking system, and me sitting in the back seat could see the movement of the pin box. I told the driver to at least stop using the jakebrake and he never did. I had to repair my frame after 3 months. I am not saying it caused the failure but it sure helped in the failure as the frame was ruined durng the trailer construction. One beam was reduced in strenght by 50% due to careless cutting, It was the one beam taking most the load of the pin box.