I've spent several decades crossing the continent. Many truck stops (e.g., Pilot/Flying J) not only have showers that you can use for a fee, but also have dump stations and potable water fills. You can also use http://www.sanidumps.com/maps/help.php and http://www.rvdumps.com to locate dump stations, and many of those are located where you can also obtain potable water. Many state parks with camping have coin-operated showers too, and you should be able to find out by just visiting the state's website for its parks. Good luck!