Complex sentence: Guido maintains this story right until the end when, in the chaos of shutting down the camp as the Americans approach, he tells his son to stay in a sweatbox until everybody has left, this being the final competition before the tank is his.
Split into simpler sentences: Guido maintains this story, even the news arrives that the Americans are coming and there is the chaos of shutting down the camp as the Americans approach. So he tells his son to hide in there and stay there until everybody has left, this being the final competition before the tank is his.