Hi Randy -- --
Don -- Is correct. The "Pin" is the same --- Commercial Trailer/RV Trailer.
The "Plate" or "Mating Surface" is larger on a Commercial Trailer -- Because of the "Verticle Loading". Hence -- A larger Commercial Hitch (Holland).
For these reasons -- The Commercial Holland Hitch will function pulling a 5th wheel RV.
The "Disc" Don referes to is a Teflon disc -- Providing the slippery substance between the "Fixed" object (Hitch) and the movable object (Trailer) -- Preventing excellerated wear/Noise/Binding.
There are 2 sizes of "Discs" -- Large for Commercial -- Small for RV.
Grease is the other medium -- Although messy -- Hence the Teflon Disc.
Don may have also been referring to a "Plate" that is fitted in the grease well and tack welded into the Commercial Hitch -- And using then the small Teflon wear ring to pull a RV 5th wheel.
Hope this helps answer the Hitch question -- --
Dave
http://community.webshots.com/album/49062992VBcrVd