When we talk about "truths that can teach the reader" from fictional stories, we are talking about themes. Even short stories can contain many themes. These are subjects, messages, or central ideas presented through the plot of the story that are universally applicable
When answering this short essay, I advise you to focus on one main theme, or lesson. When you consider the time "Rip Van Winkle" was first published (1819), a time in American history when the ideas of hard work, fighting for freedom, and gaining independence both individually and as a nation were very strong American values, this story seems to be a direct contrast to these ideas.
Rip is not just lazy. He neglects his wife (and his home duties), whom he has already driven to be an overbearing and nagging woman, because of his lack of work ethic. He escapes responsibility by fleeing to the mountains (a very classic message of "Romanticism") and sleeps through the revolution after drinking an alcoholic beverage.
There seems to be one predominant lesson here. In stark contrast to the ideals of hard work and personal responsibility presented in Benjamin Franklin's Poor Richard's Almanac, "Rip Van Winkle" seems to show the consequences of laziness, escaping responsibility, and even drinking. It could easily be considered a message to Americans (predominantly) that reinforces the ideals of the early American leaders. It also seems to be a warning not to "fall asleep" and miss out on some of the biggest changes happening in the country. It is a call to participation, certainly, but worthwhile participation. It is a call--almost directly to men--to responsibility and leadership, starting in the home and extending to society.
For more help with this question, I encourage you to check out the link below which provides more information on themes in this short story.
When writing your essay, I think there are two main themes that you could focus on when talking about truths that could help the reader. One these have to do with change. When Rip Van Winkle falls asleep, it is prior to the Revolutionary War. He slumbers for twenty years, and when he wakes up, he misses the entirety of the war and the social change that comes with the revolution. In front of the new hotel, no longer is there a picture of King George, but rather, there is an image of George Washington. People have now adjusted to living under a republic and a new way of life. Not only that, but his personal life is not as he used to know it. His wife is now deceased, his daugther has been wed, and he has a grandchild. Basically, the book makes the case that change happens, and you have to adjust to it.
Another talking point could be about the need to work hard. Though Rip is likeable, he's not someone who could be relied upon to put in a lot of effort. This could be seen in the poor condition of his farm and why his wife always nags him. Avoiding his wife and work explains why he goes up into the mountains and encounters a group of men playing ninepins. They offer him a drink, which leads to his long sleep. Because of his failure to deal with the hardships of life and do work, he missed out on being able to be part of the Revolution and help change society. The message here is that while we would all like to sleep our troubles away, we need to work hard to make change happen.