Weighed my trailer the other day, the pin weight comes in at 12%. Had the fresh tank full which resides behind the axles. An educated guess is the pin will fluctuate from the low of 12% to maybe 17%...
Dropping the trailer ON the scales would do nothing for telling you pin weight. That would only tell you total weight of camper and what weight is on the front legs if on a different section of the scale which is different than pin weight.
Easy to do the right way. Weight full loaded combo...pull off...drop trailer...weigh just truck and subtract differences.