The reason why Stanley's Camp Green Lake Experience was ultimately successful was of several reasons. One was because of how much he learnt during this time. For example, he learnt if you don't give up you can achieve a lot. He didn't give up when he was walking toward the big thumb; if he did then he would be dead. He didn't give up when he was digging Zero's and his own hole. Stanley also learnt that not all camps were like "Camp Fun and Games".
During the time at the camp, Stanley succeeded - without realising - in loosing weight. He also built stamina and strength. His body learned to cope with the extreme heat and water shortage (not forgetting when Stanley was deprived of water by Mr. Sir). Stanley's hands became tough from all the digging and he became tough to the bullying of the other boys and councillors.
But he wasn't bullied by one person - Zero; who became Stanley's first friend. A friend which Stanley would save from death.
Throughout the time at Camp Green Lake Stanley discovered many different things, one of which was the long time riddle of his great-grandfather. His great-grandfather said "I found refuge on gods thumb". His family always wondered how he survived in the desert without food or water for three weeks. Even after he got better, he himself didn't know what he was talking about. But Stanley found out that he took refuge on gods thumb by living on the mountain which looked exactly like a humongous thumb. On "god's thumb", you could find water and food; onions.
Stanley also broke the curse which had been on the Yelnats family for several decades. The curse was broken by having the great-great-grandson of Elya Yelnat's, carry the great-great-great-grandson of Madame...