Veteran of two years of actual Mexican food truck experience. Over that time I've failed too often and learned more than a person should really have to. =^) Mexican food is extremely prep-intensive. Do everything you can to spend less time at food prep and more pushing food out the window. Slow cooking does not lend itself to this.
I've done slow cooking with pork carnitas and with puerco al pastor. And I make excellent Mexican. My carnitas take all night and melt in your mouth. It never paid.
Fish tacos sell well and don't take so much prep. Chicken asada if you get the recipe right. I'll tell you if you want. It sells and it cooks fast. If you are going to cook pork, either cut the good bits out of a butt and grill them at order time for tacos (time consuming) or put it in a pressure cooker long enough that it's well past tender. Hit it with a potato masher, quickly pick out the gristly bits, season it, and serve.
Get your ticket average up over $10-12 with combo specials or you won't even come close to surviving, higher in the expensive locales. I mean no disrespect here: cater to anglos. Get into the toniest venues you can and find a way to present your cuisine as edgy (most anglos have never had authentic Mexican food and they find it edgy to begin with so phew). Hispanics will not pay $10 for two tacos and a drink but that's what you gotta sell.
Do the grilled onions, radish and roasted jalepeno garnishes if you can. White people find this entrancing. Carry the Mexican Coke in bottles if you can, again, white people think Mexican Coke is a revelation and will pay $3 for a ten ounce bottle once word spreads. Display Mexican canned drinks if you can, they are exotique.
See what I'm doing with my unsolicited advice? Find a way to present traditional Mexican food as avant-garde to gabachos, or else you will be competing with roachers who are happy to sell mediocre tacos for 99c.