To turn the taco meat into sloppy joes, I'd probably just add tomatoe paste, a bit of water and maybe some brown sugar to give it that "sloppy joe" flavor.
Ideas for other meals? Most of my ideas would have to be made off site and brought prepared.
Hot dogs. If you set them upright in the crock pot, you can cook them just as they are. You don't have to add water or anything. I do for birthday parties all the time. It works great.
Fried rice. Obviously, you'd have to make it off site, but you could warm it in crock pots. But you can add frozen veggies and any kind of meat, or go with out meat (the egg will be protein already).
Pasta dishes. Again, cook offsite and warm there. Pasta dishes are very affordable and pretty easy to prepare. They can even be made very far in advance and then frozen, to be used later.
Bean and rice can make burritos.
Egg salad. Eggs are really inexpensive. Mayo isn't too bad either, especially if you find coupons or shop wholesale.
Baked potatoes in the roaster oven.
If you keep an eye out pork shoulder roast often goes on sale for a great price. Cooked in the crock pot, these make great pulled pork for sandwiches.
Chili. There are vegetarian recipes available if meat is not in the budget. This can be prepared in the crock pots on site. It's also another recipe that can be made ahead of time and then frozen until you need it.
If you have an electric skillet...grilled cheese sandwiches (and possibly tomato soup too)
Soups are very inexpensive. Especially noodle or potato soups.
If you google "ramen noodle recipes", you'll find a ton of ideas. I know ramen noodles are questionable because of the msg. The Msg is in the flavor packets, not the noodles. So I am sure you can find recipes that do not use the flavor packets.
This trick won't be handy now, but around the holidays, stock up on turkeys. Whole turkeys are often free with grocery store purchases. Then you'd have a ton of options.