S05E23: Uncle Grandpa: The High School Years

In the series finale that's a flashback that shows the gang in high school, Uncle Grandpa, Pizza Steve, Mr. Gus, Tiger, Frankenstein and Belly Bag are all in detention and while the principal is using the bathroom, they all try to find ways to fight boredom and slowly start to build a friendship. After a fight, they all make up and Uncle Grandpa suggests they all live together and have fun adventures later on in the RV.