The typical RV toilet does not have a tank. Water for the flush is usually activated by a pedal on the side near the floor. Partial depression of the pedal will get water flowing usually with many jets with rather complete rinsing of the sides of the bowl. Full depression of the pedal will open the flapper to empty the contents of the bowl. Hence, IMHO, an RV toilet usually uses less water than even a "water saver" toilet.