One of the smartest additions I made to my rig was a 5-gal water cooler, and Igloo ice maker that sits on top. It is the source of all my drinking & cooking water... and I'm one of those guys who always has a mug of ice water handy.
Now, that said... if you need water in the tank for the toilet, shower, kitchen sink, etc... many Interstate Rest Areas have a hose available. It not, any truck stop that accommodates RV's will have a hose you can get potable water from.