As an update, (and also as a warning to others who might be considering it), I went to the Rigid RC site link, and bought the sliding/oscillating fifth wheel here:
http://www.rigidrc.com/shop/fifth-wh...114-truck.html
Delivery took about a month, and I just installed it today. The quality of the parts if first rate, and the slider works smoothly and locks in position securely. My issue with it is the dimensions of the kingpin and fifth wheel slot. The Tamiya kingpin does NOT fit this fifth wheel, (it is too tall and the diameter of the head is about 0.050" too wide). I also tried a Scale Art kingpin, and that does not fit this fifth wheel either. The Wedico kingpin is far too large to work with it, and I don't have a Robbe or Carson trailer to try on it.
So, while it IS good that they enclose a kingpin that works with their fifth wheel, what it means for me is that I can fit up ONE trailer to work with it. A trailer so equipped won't work with a Tamiya fifth wheel on another tractor, and I won't have the option of using other trailers with this tractor. I can learn to live with that, but am just disappointed that Rigid RC didn't design it to work with a Tamiya-size kingpin.