There's a hot dog truck I go to periodically, that does fairly well. They offer more than a hot dog cart, in that they have like 3 different sausages you can order, and they're griddled rather than steamed. Anyway, I know they keep things as simple as possible, by getting most of their supplies from Costco, and offering a limited selection. I think you can get your sausage, a bag of chips, and a wide array of sodas, and that's it.
Service is quick, because most of their business is regular hot dogs, and they have enough experience now to be able to make more or less the right number of other sausages.
My understanding is it's a living, it gives them freedom, and enough money to more or less last through the winter without having to work. I think they said they work about 6 hours on most days; an hour to set up, 3 hours selling, and 2 hours cleaning up/setting up for the next day.
They did run into a problem about a year and a half ago, where the building in the parking lot they were using got leased to a new restaurant, so they had to move. They crossed town lines and got a food service permit for the new town, but the new lot they were operating in wasn't zoned for such a thing, so they had to scramble for a new location. But they ended up in an even better spot now, even though it's no longer close enough for me to walk my dog to it (they keep all the hot dogs at the end of the day to give to dogs they see, which I liked).