Abstract: A combination fresh water supply and waste holding tank for use in mobile structures such as vehicles. The tank includes separate half shell portions and an extensible divider wall extending across the internal tank space between the half shell portions. A fresh water supply holding structure is provided in one portion of the shell spaces defined by the divider wall and a waste holding structure is provided in the other portion of the spaces defined by the divider wall. Each of the divider wall, fresh water supply holding structure and waste holding structure are extensible whereby selectively substantially the entire internal space of the shell may be occupied by the fresh water supply holding structure and fresh water therein, or substantially the entire internal space of the shell may be occupied by the waste holding structure and waste therein.