Jackson simply means that he's very good at being homeless. Some people are good at their job; others are good at sports; and still others good at arts and crafts. But Jackson's good at being homeless. In fact, as Jackson reflects, it's the only thing he's ever been any good at.
What makes Jackson a particularly effective homeless person is his unrivaled ability to source regular supplies of free food. He always knows how to get the best free food, a very important skill for someone in his position. Not only that, but due to his friendly relations with restaurant and convenience-store managers, he's able to use employees' bathrooms.
Jackson's rather proud of this because it shows that he can be trusted. Many people won't give a homeless person the time of day, but these restaurant and convenience-store managers clearly have enough faith in Jackson to let him use their facilities.