The smoother ride location will be highly dependent upon the rig. I suspect the closer the weight is to the max weight the smoother the ride. The ride above the truck bed will be rough if the truck has heavy duty suspension and there isn't much weight on the axel; the same for the trailer axels.
We found this to be true for the two TT we have had. The first had a smooth ride inside based upon how little movement there was for items left on the counters. The same is not true for the current TT. Our first TT was closer to the GVWR than our current trailer.