For the record, water weighs 8.35 pounds per gallon.
That said, we try to fill up at a water source that we would be happy drinking. Sometimes campgrounds have great wells, some are on city water and we carry a full tank most of the time.
Bottled for drinking.
Twice a year I throw in a couple of cups of bleach and drive it around, drain, rinse, and fill.
Twice on holiday weekends someone has hit a water pedestal and they've had to turn off the water to the entire campground to fix it. If you don't carry water, you're...dead in the water.:E
Happy trails!