GVWR is the max load, so all weight is figured, including full tanks. On RV trailers, the GVWR varies greatly with the running gear, axle, springs, and brake maximums. I have seen the same trailer, with upgraded axles, from 4,400, to 5,200 lb and 2,000 lbs more GVWR.
Rare would be the time when all tanks were full, as most dump ASAP. If boondocking, can't dump, you will only have the water you brought with you, moving into the waste tanks.
BTW...How much does 78 gal of black tank waste weigh?
Jerry