James DiSabatino didn't set out to build one of Boston's biggest food trucks. In fact, he didn't have much of a plan at all. He just knew he wanted to sell grilled cheese, and a food truck was a relatively cheap way to do so. Since then, his food truck, Roxy's Grilled Cheese, has expanded into multiple brick-and-mortar locations. He's building an arcade in partnership with another local joint. He's even expanding out of his grilled cheese niche with an all-new restaurant theme. How'd he do it?