You should fill your fresh water tank as it is meant to provide water for washing and the toilet, plus other uses. Shouldn't add too much weight to your overall load. We also haul fresh water jugs or bottled water in our tow behind Jeep so we don't have to worry about water supply in case we are in a CG w/o water or we want to boondock. Sanitize the FW tank with bleach and rinse out the tank before filling for the road trip. Add a little bleach to the tank when you fill it, maybe a half cup for 20-30 gallons.