gray and black tank
I wouldn't be concerned on the weight of the Gray & Black tanks, unless you keep filling the fresh and never empty the gray or black.
In other words as the fresh gets used up it gets lighter, the gray and black tanks then get heavier.
I don't know of anyone that travels with full fresh, gray and black tanks.
There are also plenty of dump stations available including some rest areas and gas stations, also most campgrounds charge a small fee to just dump ($5-$10).
So usually black & gray weight are not issues