Yes, you DO need to remove your fifth wheel hitch. All you have to do is pop a few latches and it'll lift free (do you have a bucket tractor?
) Yes its heavy bit its what you need to do. Then you just slide in a gooseneck "plate" which bolts to the same rails you have that the existing hitch latches to. Not including the lifting part, its a five minute job, no tools required.
You have a very expensive hitch installed there now... I wouldn't suggest replacing it with anything. Nor do you want to add any type of adapter to the hitches to get them to be cross-compatible. Yes, there are several options, but none of them are designed for the stress of what you are towing. That massive hitch you have is (I'll wager) rated for 12-18k weight, with 18k being VERY high. A gooseneck, ANY gooseneck, is rated for 20-25k, and sometimes as high as 30k. That's a very different thing. Its just how the hitch works, and where it puts stress. Plus, goosenecks need safety chains, and fifthwheels don't. So if you use an adapter of some sorts, you will not have anchor points for the safety chains.
There are other systems, if you want to totally remove what you have now. You can get a B&W hidden hitch, which is a gooseneck hitch that allows you to remove the ball and add in a hitch called the "Companion hitch" which is a fifthwheel that attaches to the same anchor hole the gooseneck does. But either way, a gooseneck and a fifthwheel are two totally different hitches, and need to stay that way when it comes to towing a trailer. You need, yes, need, to have the correct hitch from the ground up. DO NOT USE ADAPTERS. I know its not what you want to hear, and I'm sorry, but to be safe its what you need to use.
If you modify the trailer to be towable with a fifthwheel, you run into several issues. One, the weight can easily exceed the hitch's capacity. Two, the stress points on the trailer's hitch are different. Three, you said you have a tall truck as it is. Goosenecks are designed to hitch into the BASE of your bed. Fifthwheels hitch two feet higher, at the top of that massive hitch. That means your horse trailer will be 2' higher in the nose, leading to a VERY steep angle (which should be perfectly level). Four, you will loose your safety chains. Five, NOBODY ELSE WILL BE ABLE TO TOW YOUR TRAILER!! This may be considered a plus, for theft, but you can buy a very nice hitch lock for under $100. And if you need to in a pinch, only your truck will be able to pull that modified trailer. So please, don't do it! It really is easy to remove the fifthwheel hitch. The only pain is manhandling it, and if you have a winch or tractor, that becomes easy too. The gooseneck plate snaps right in using your existing hardware, and has everything you need.
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