Place a medium skillet over medium heat and add the oil. Once hot, cook the onion, jalapeno, and garlic until the onion is translucent, about 4-5 minutes. Add the tomatoes and cook for 2-3 minutes. Season with salt to taste.
Coat the crock of a slow cooker with non-stick cooking spray. Add the sauteed vegetables to the crock along with the green chilies, shredded cheese, cream cheese, and 1/2 cup milk. Stir to combine.
Cover and cook on LOW for 1 hour. Stir to combine and cook for another 30 minutes or until the cheese is melted and bubbly. Add more milk, 1 tablespoon at a time, if the dip is too thick.
Serve immediately or turn the slow cooker to WARM for up to 3 hours, stirring occasionally. Serve warm with tortilla chips.