X______________Y <-- Travel Trailer
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X______________Y <-- 5th Wheel
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So above I made 2 teeter totters (hopefully the font keeps everything where I need it. On a trailer, you have a situation where your load "Y" is put down after the fulcrum (rear axle). So when you load "Y", you take some load off of "X", therefore you get the teeter-totter reaction of lifting "X", however you typically don't have much of a leverage arm so you don't remove an equal amount of weight from "X".
On the second teeter totter, the fulcrum should be right under "Y". So if you keep loading "Y", no matter how much, there's no leverage arm so you will not lift "X" up by any amount. Go to your local community park and try it. You can stomp down all you want on the fulcrum, but you're not going to raise "X" up by any means. This is why you don't need weight distribution on a 5th wheel.