Well, if there is one thing that people will always do, it's eat. Horse show people love health food too, they're usually more conscience about eating right and don't want heavy meals. Sounds like a great idea, good luck.
Just some thoughts.. check state regulations you might have to have a food saftey course yourself as well as having the vehicle your serving from inspected. Be vigilant in keeping an strict eye on your overhead expenses, they are what will kill a business. Have a clear vision of what you want to do but be ready to adapt. And remember the old saying, you dont own a business the business owns you.
My first business was in 2nd grade. We lived on a lake and I sold worms by the dozen to fisherman. My uncles built my worm box for free (no start up costs) I washed out milk jugs for packaging(no packing costs) I ammended the worm soil with coffee grounds donated by a resturant( they liked me I guess). I would go out all night picking worms by flash light when all the other kids were playing.(free merchandise). Well, I had a pretty kicking business. I made more than any kid my age should have. Great experience, been in some kinda business ever since.
Best of luck to you. Think it out, then go for it.
And it isn't just horse shows that food trucks frequent. Race tracks have them! Lone Star had food truck days 2 or 3 times last year. Santa Anita has them all the time. When we were there last spring their must have been 30 in the infield. On the other hand, Rolex for instance, one company has an exclusive contract. I would imagine that's the case for most big show venues. Too bad because food at Rolex 20 years ago was so much better!
"Everyone will start to cheer, when you put on your sailin shoes"-Lowell George
Definitely a promising idea, but do a lot of research! Some (annoying) places prohibit them, and/or make the sanitary guidelines they have to follow so insane that it is difficult to do business. Even if you don't think you need one, I promise that the time spent to research your market, etc and put together a really well thought out business plan will absolutely pay off.