Notice nobody here is really making a big argument for mpg difference being a major consideration. As most have mentioned, they didn't notice the difference. All so called mpg experts, everywhere, says weight makes a difference. They just won't say how much ( or how little ;).
RVs with smaller CCC, then the extra water can make a noticeable difference. WHEN I start traveling anticipating dry camping, I will haul 1/4-1/3 tank for bathroom use. I will have to cut back more on the stuff we have previously packed in the TT.
I have previously used the cheaper in line water filters for CG use and now have a better quality one.
I, too, haul filtered water in jugs for drinking & cooking. I have found out how much I need per day average and cut back on those jugs, again to save CCC weight. When traveling for extended time, I will use my Berkey style drip filter for clean, fresh water.
My wife treated stocking the TT like a cabin on the lake. After my CAT scale weighing, we cut back significantly and keep aware of not taking a lot of extra nice to have, just in case, but seldom or never used stuff. Last camping trip was our most easy going, laid back, and we packed about half as much as before. Last year was our first year with a TT.