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%...
You said fresh water tank was behind axles. A full tank (28 gal per website) should weigh in at something like 230 lbs. Sounds reasonable to me that it would take some weight off of pin. I think your worried for no reason. You could always take some water out of tank.