CAIRO, May 28 (Reuters) - Israeli tanks shelled a tent camp in an evacuation area west of Rafah and killed at least 21 people on Tuesday, Gaza health authorities said, while advancing to the heart of the southern Gaza city for the first time after a night of heavy bombardment. Two days after an Israeli airstrike on another camp stirred global condemnation, Gaza emergency services said four tank shells hit a cluster of tents in Al-Mawasi, a coastal strip that Israel had advised civilians in Rafah to move to for safety.