I only travel with around 8 gallons. no need to bring anymore weight than I need. besides we almost always get FHU. when we do get electric only I fill up at the campground. only takes a few minutes and is very easy.
And also, some tanks are mounted way up front or in my case at the very rear of the trailer. this will mess with your tongue weight if you don`t always travel with full water. for me a full freshwater tank is 44 gallons! if I only travel with approx. 8 gallons that saves me 300lbs. that is a good amount of weight savings!