Well, to answer your question....most gas stations will have a water supply. As will almost all campgrounds (even when they don't have full Hook-ups)...unless you are planning to camp at a Wilderness Site without any water.
One of the advantages of filling your tank before you leave.......you know what your water tastes like! Not all water tastes the same, or smells the same, and that may be an issue.
Traveling with a full tank probably won't change much of the mileage....it's gonna be bad no matter what:) Are you really excited because your mileage went from 7.25 to 7.3 mpg? Depending on just how you drive, you can probably make up the difference just by slowing down 5 mph.:B
Have a great trip.
