One thing that the salesmen may have been alluding to is that some manfs spec the axle
(which is generally on what the GVWR is based) with plenty of capacity above the dry weight
of the trailer and others spec it to be much closer to the dry weight
. This is likely what they were talking about. Without some scale weights of real rigs, it's hard to know what the personal load capacity actually is.
Without getting actual scale weights (hopefully with all travel gear and fluids loaded), the next best conservative estimate is obtained by using the GVWR (the assumption here is that the trailer will never exceed that weight), plus a 15% estimate for the tounge weight
(TW is seen as a load on the TV, whereas GVWR is seen as towed weight).