When we went to THE RALLY (Good Sam national rally) in Albuquerque NM in April we went 6 days with just 30 amp electricity. We started out with empty holding tanks and a full fresh water tank. At the end of THE RALLY our bathroom gray tank read 1/3 and the galley tank E and the black tank 1/3 and the fresh water still read full. We did not waste water but we took a quick shower every day, used paper plates and drank bottled water. We used the toilet as needed but took advantage of restroom facilities while we were out and about. You should have no problem with just 3 days of use. Just be sure they are empty when you start out.