Do you mean a big trailer, like a catering or "taco" truck?
As a consumer, I recommend you pick a few things to do really well as opposed to trying to have too much variety and settling for mediocrity. I believe there is such a thing as a "loss leader" in food. Go ahead and make the biggest burger with the best quality meat, and I will go out of my way to go there and bring my friends with me. You may not profit much off the burger, but you have 4 meals with drinks and sides so it evens out.
Get unusual sodas or candy…Stewart’s instead of only Coke products. Browse the snacks and sides sections here. If I really need a Moon Pie at lunchtime, I’ll pass 3 other trucks to get to yours. Plus, when the students/office workers are deciding where to go, you will stand out because they will have a way to reference you…"Let’s go to the truck that has the Zapp’s potato chips!"
If you park in the same place, make an effort to remember people’s names and favorites. It makes the customer feel really good. If you are in a Mexican-American area, learn a little Spanish, near the school for the deaf, learn a few signs.
I am assuming you are talking about a lunch truck. I would want very different things than what I would want from the truck that sits outside the bar at 1am.